Upgrading your PC's processor isn't the easiest or cheapest way to boost its performance, but it can deliver dramatic results. The right chip can transform a sluggish machine into a powerhouse.
The practicalities of fitting a processor have remained largely unchanged since the early days of PCs, but selecting a new chip isn't so simple. Once it was just a matter of picking the one with the highest GHz rating. Now you've got to keep that in mind, along with an increasingly complex set of other variables.
First, you need to look at the CPU socket on the motherboard to see whether you need an Intel or AMD processor. Next, you need to consider the processor itself. Each manufacturer will have a slew of subtly different technologies, all packaged inside chips with names that tell you little about them.
When you've chosen a chip type, you'll need to weigh up how many processor cores you need. Should you pick fewer faster cores or go for more cores that run at a comparatively slow speed? Then there are nuances like automatic turbo modes and hypertheading to consider.
Finally, you need to think about the interplay between your CPU and graphics processing unit.
Lets consider one factor in detail: the number of cores. As perverse as it sounds, more cores don't necessarily mean more speed. Speed is largely determined by the applications you're running, and how they use those cores.
Thankfully, many video editing suites and games are now written to use multiple cores simultaneously. The same applications (including Photoshop, Adobe Premiere and HD video players) can take advantage of GPU acceleration. You therefore need to think about your system holistically, considering your requirements.
Video-editing professionals, for example, might opt for a dual-core MacBook Pro, with a GeForce chip for encoding HD streams with Final Cut Pro. This would do a better job than a PC with a quad-core CPU and a non-programmable graphics card.
There are other factors to consider, too. For example, Intel's new Sandy Bridge CPUs have an astonishingly good Quick Sync video encoding engine, which is often better than GPU acceleration.
We'll guide you through the business of picking the best chip. It might not be an easy decision, but with the array of new silicon, you're sure to find one that's a good fit for you and your system.
The great processor war heats up
The battle between AMD and Intel has been raging for decades, but it feels like Intel may have gained the upper hand. Even the billion-dollar recall of its new 6 Series motherboards failed to upset the launch of its Sandy Bridge processors - the week the recall was announced, Intel's share price actually rose. Such is the market's confidence in Intel's silicon.
AMD has been trailing in raw performance since the launch of Intel's Core 2, which blew the Athlon 64 out of the water. Throwing cores at the problem hasn't helped either - six-core Phenom II chips can't keep pace with their Intel quad-core rivals.
Don't write AMD off just yet, though. This summer we're expecting to see AMD processors based on a completely new design, currently called Bulldozer, which could pose a real threat to Intel's dominance.
Bulldozer's design looks very advanced and efficient. AMD has yet to confirm which chips it's planning to launch, but the internal structure is very different from what's gone before.
The basic unit within Bulldozer is a dual-core module, which can share resources across its two execution engines or divide them up for increased parallelism. The first Bulldozer chips, codenamed Zambezi, will be come in four, six or eight-core variants, which suggests they'll build on AMD's current strategy of offering more cores for less than Intel.
The biggest problem AMD faces will be branding. It's already begun its Fusion project, which - like Intel's Sandy Bridge chips - integrates a graphics core onto the same die as a CPU. Bulldozer, however, won't be part of the Fusion line-up for over a year.
AMD's first Fusion processors have been built around the low-end Bobcat core, which is aimed at netbooks and thin and light laptops - somewhere above an Atom, but below a Core i3.
The first Fusion chips for desktop use will be based on Phenom II, and will launch around the same time as Bulldozer. That gives AMD the problem of trying to communicate the relative strengths and weaknesses of many different CPU types at the same time.
By comparison, Intel has already begun retiring many of its Nehalem-based Core processors, giving it two main platforms - Atom and Core - which are relatively easy to understand. Pentium and Celeron will continue to exist as value brands, but even these will use Sandy Bridge designs and the new socket 1155 motherboards.
Only at Intel's extreme high end, where six-core CPUs based on the older Gulftown design are still being released, is there any inconsistency - and these are already starting to look irrelevant in the wake of Sandy Bridge's performance (see the 980x review).
By the time the new Bulldozer Fusion chips arrive, Intel should have completed its plans to migrate Sandy Bridge onto a 22nm process. With that sort of performance increase and cost reduction, AMD could be left trailing behind.
12 CPU choices
Intel processors: The high end
Intel high end

Intel Core i7 980X 
Price: £750
Want the finest CPU money can buy? Look no further than the Intel Core i7 980X. It's the undisputed world heavyweight champ among chips. Just remember you'll need a lot of it. The money, that is.
But what a processor you get in return. The Core I7 980X is, of course, a six-core beast. But this is six-cores Intel style, so that's two threads per core and a dozen of those little green graphs when you fire up task manager. That's unparalleled, er, parallelism in a PC processor.
In that context, even AMD's Phenom II X6 1090T Black Edition looks rather ordinary. It may have six cores. But each is single-threaded only. Moreover, AMD's underlying CPU architecture is pretty ancient while the Intel Core i7 980X is literally the latest thing, right down to is impossibly tiny 32nm underpinnings. What it isn't however, is unique. The recently released Core i7 970 is largely the same six-core, 12-thread processor at a slightly less offensive price point.
Verdict: 4.5/5
Intel Core i5 2500K 
Price: £190
Ready or not, here they come. Intel is rolling out a thoroughly overhauled range of PC processors based on its new Sandy Bridge microarchitecture. Our first taste of the new chips comes in the form of the Intel Core i5-2500K and Intel Core i7-2600K desktop CPUs.
Thanks to the baffling array of chips, sockets and brands, we've barely got to grips with Intel's existing CPU range. Certainly Intel's main rival, AMD, has no answer in outright performance terms to the chips Intel already offers, but the relentless march of technology must go on.
So, ignore the familiar Core i5 and Core i7 branding. These are all new processors and they're ready to roll.
As it happens, Intel could actually do with more powerful and, crucially, more power efficient processors for laptop PCs. Deep down, that's what Sandy Bridge is really about. However, as we'll learn, Sandy Bridge has a lot to offer for the desktop, too, including exciting new features such as a hardware video transcoding engine and much-improved integrated graphics.
Verdict: 4.5/5
Intel Core i7 2600S
All Rounder 
Price: £235
Another day another new Sandy Bridge CPU, and another new suffix to get your head around. This time it's the Intel Core i7 2600S.
Many people have heard of the Core i7 2600K by now, the unlocked overclocking demon that was part of the original Sandy Bridge launch, but the Core i7 2600S you may not have heard of.
It's also pretty well known that if a second-gen Core CPU doesn't have a K at the end of the model number, its pretty much game over for any sort of serious overclocking. So what does the S stand for, and does it mean even more features turned off?
Well no, the S models represent more low power units, but just not as low powered as the T models. So instead of the 95W of a standard 2600 part, they have a TDP of 65W, as compared to the 45W of the T series.
Apart from the low power rating, the Intel Core i7 2600S still retains all the familiar features of the 2600 family: four cores, eight threads and 8MB of Smart Cache. But as with all the S class chips, it's clocked slower than the rest of the their family.
In the case of the Core i7 2600S, this means it runs at 2.8GHz compared to the 3.4GHz (3.8GHz max Turbo) of the i7 2600 and Intel Core i7 2600K.
Verdict: 4/5 
Intel processors: The mid-range
Intel mid range

Intel Core i5 2300
Price: £150
This is the lowest specced of Intel's new Core i5 series, but it only costs a little less than the faster 2500. That said, there's not a lot of difference in performance either unless you're planning to overclock, so you may as well opt for this slightly cheaper model and save a little cash.
This quad-core Sandy Bridge CPU has a base clockspeed of 2.8GHz, and can accelerate to 3.1GHz under Turbo mode. Like all i5s, it doesn't have Hyperthreading enabled, but it's still more than a match for all but the very fastest of AMD's six-core Phenom IIs in everything except demanding video encoding applications.
Even though it's Intel's least expensive quad-core, the i5 2300 still isn't cheap. Factor in the price of an H67 motherboard, and it highlights a significant problem for a lot of the current Core line-up: there are a lot of three, four and even six-core AMD processors available for less, and if you're on a really tight budget there's every reason to be swayed by them.
Verdict: 4/5
Pentium G6950 
Price: £75
This the cheapest CPU currently available from Intel, costing just £75, but it still offers an impressive balance of performance and value.
Its low price can be attributed to the fact that it's not a new chip. Launched last year and based on the Clarkdale architecture, the Pentium Dual Core G6950 is almost identical to the outgoing Core i3s in that it requires a Socket 1156 motherboard, has an on-board graphics accelerator (albeit a low power one) and is designed around the superb Nehalem core.
With a base clock of 2.8GHz and no Turbo Boost or Hyperthreading abilities, the G6950 can't keep up with its Core i3 brethren, but it's proved itself to be a top overclocker in the past.
Ultimately though, if you're looking at spending less than £100 on a new CPU, you'll probably be more attracted to the even cheaper Athlon IIs. They aren't as fast as the G6950, but they'll get the job done perfectly well for less.
Verdict: 3/5
Intel Core i3 2100 
All Rounder 
Price: £108
After all the Sandy Bridge goodness without the quad-core price-tag? Then the dual-core Intel Core i3 2100 might well be up your street.
While the high-end unlocked Sandy Bridge CPUs, the Intel Core i7 2600K and Intel Core i5 2500K were rightfully taking all the plaudits for being overclocking monsters, the 2600K especially, not many people were looking at the other end of the food chain.
That is to say in the value end of the market where the lowly Intel Core i3 2100 is to be found. As with all the current Sandy Bridge processors, it's built on the 32nm process and manages to pack 504 million transistors into its die.
The Core i3 2100 is clocked at 3.1GHz with 3MB of L3 cache, which sounds like it should be a fairly blazing chip. However it has no Turbo Boost and is totally locked down, so there's no overclocking fun available on the processing side.
This is a pity, because some of the best overclockable Intel chips in the past have come from this segment of the market.
You may not be able to overclock the CPU core but you can though do a smidgen of tinkering to the HD2000 graphics core integrated into this second generation Core CPU.
Verdict: 3/5
AMD processors: The high-end
AMD high-end

AMD Phenom II X6 1100T BE 
Price: £208
This new hex-core chip is symptomatic of AMD's current predicament: that its most expensive PC processors sell for barely one-third the price of Intel's.
And that's not a situation the new AMD Phenom II X6 1100T Black Edition is going to change – despite the fact that it's officially AMD's fastest and most expensive chip.
With a retail sticker around £215, the AMD Phenom II X6 1100T Black Edition is priced on a par with the very cheapest of Intel's Core i7 processors such as the Intel Core i7 870. Nevertheless, it serves up six execution cores to the 870's four. Six-core Intel CPUs are far more expensive, starting around £700.
Then again, Intel's cores do much more work per cycle. Until it releases the long awaited Bulldozer CPU architecture, AMD needs to sell more cores for less cash. In the meantime, this revised six-core Phenom II X6 raises AMD's game incrementally with an increase in clockspeed from 3.2GHz to 3.3GHz.
For the most part, the AMD Phenom II X6 1100T Black Edition is not a new processor. It's based on AMD's increasingly familiar six-core Thuban die, a chip that can trace its roots directly back to the AMD Hammer CPU architecture first seen in 2003. Thus, it's a 45nm CPU with 512k cache per core and a further 6MB of shared cache memory.
Verdict: 4.5/5
Phenom II X4 970BE 
Price: £140
By now, we were hoping to able to get our hands on AMD's new Bulldozer archtecture processors, but there's still no sign of them, so instead we back with the familiar Phenom II theme. Give it up for the new AMD Phenom II X4 970 Black Edition.
Based on the 45nm Deneb core that's being doing duty in quad-core Phenom II chips for nearly two years, the AMD Phenom II X4 970 Black Edition sports a heady stock clockspeed of 3.5GHz. Yup, that's precisely 100MHz or three per cent faster than the Phenom II X4 965 Black Edition, previously the fastest X4 model.
This kind of incremental upgrade is symptomatic of an ageing processor architecture on it last legs. But while Bulldozer can't come soon enough, there's no reason why the 970 shouldn't be an attractive CPU. It all comes down to pricing and positioning. At £140, the 970 squares up directly against Intel's Core i5 760. Game on.
Verdict: 4/5
Phenom II X6 1055T 
All rounder 
Price: £140
Remember when AMD launched its first quad-core processor in 2007? We can, because back then it seemed like AMD was months from keeling over stone dead. Today, the company is in much finer fettle. For proof, look no further than the new AMD Phenom II X6 1055T.
Somehow, AMD has managed to produce a six-core PC processor and sell it for just over £10 more than its best quad-core chip, the Phenom II X4 965 Black Edition. It's actually cheaper than several Intel quads. Inspect the detail specifications and the Phenom II X6 1055T only gets more impressive.
The transistor count has grown from 758 million transistors to 904 million. And yet the smaller quad-core 965 is rated at 140 Watts while this new six-core 1055T is a 125 Watt chip. Of course, at 2.8GHz, the 1055T is clocked quite a bit lower than the 3.4GHz 965 BE. But AMD has clearly done something right.
Verdict: 5/5
AMD processors: The mid-range
AMD mid range

Phenom II X4 955BE 
Price: £107
When AMD rolled out its chips for the new AM3 CPU socket, we were baffled. AM3 dragged AMD into the DDR3 era. But with that in mind, why were the first AM3-compatible CPUs groveling models with cut-down caches, lower frequencies or fewer cores?
Surely the new socket deserved a brand new processor to showcase its beefed up bandwidth and multi-core majesty? Finally that chip has arrived in the form of the new Phenom II X4 955 Black Edition.
For those of you without a double first in cryptography, allow us to decode the ludicrous product name. For starters, it's a Phenom II chip and that means cool-running, high-clocking 45nm silicon and hence not AMD's utterly rubbish 65nm transistors. X4, of course, indicates four cores, which is as good as it currently gets.
Verdict: 4/5
Athlon II X2 260 
Price: £50
Want to build a cheap media server? You may have been tempted by one those bare-bones net-tops with a dual-core Atom processor and an Nvidia video card, but why would you go for one of those when it's easy to build a system around this chip for less than £250?
Unlike the Atom, the dual core Athlon II is a 'proper' processor that supports out-of-order execution. It'll handle most games and multitasking, and doesn't suffer from the same grinding slowdown as an Atom.
The fact that it isn't as power-efficient as Intel's mobile marvel isn't such a concern if you're after a low-end desktop. In fact, the difference in overall performance between this and the Phenom II X2 565BE won't be tangible to most people.
It sits between five and ten frames per second behind its stablemate in games, and seconds (rather than minutes) behind in video encoding. It's one of the best ways to build a silent, low-cost second system for simple tasks. The question is, are you sure you want to?
Verdict: 4/5
Phenom II X2 565BE 
All Rounder 
Price: £90
Whatever happened to the triple core X3 processors that AMD was hoping would win over the sub- £100 crowd? They seem to have sold out or vanished from most of our favourite online retail stores, leaving us with this twin-cored chip as AMD's best performer at that price.
The X2 565BE certainly looks like good value for money. A high 3.4GHz clockspeed and unlocked multiplier make it a solid performer for older games. The problem is that, overall, it doesn't convince us that it's worth saving the £17 compared to the quad core X4 955BE, which is almost twice as fast for video encoding and has the framerate advantage in the increasing number of well threaded games.
Intel's lowest priced retail CPU, the Pentium G6950, will turn in better performance for less too. If you're after some real bargain silicon - for a silently-running second machine, for example - then you should be looking at the cheaper Athlon IIs instead. The 565BE is likely to be a disappointment.
Verdict: 2/5