Lab

Processors

Whether you want to give your current PC a new turn of speed or you're planning a completely new computer, it is important to know which processor is right for you. Choose badly, and you could waste hundreds of pounds - or end up with a computer that isn't up to the job.

This month, we've reviewed every single type of desktop processor produced by the two market-leading manufacturers, AMD and Intel. These range from budget processors that are ideal for a simple, low-cost computer to top-of-the-range powerhouses that will satisfy the most demanding user. Several are dual-core chips that give you the power of two processors in one, making them ideal for tasks such as video encoding. Others are designed to provide the most speed possible for 3D gaming.

SELECTION PROCESSOR

These processors range in price from £51 to £765 including VAT, but be warned: paying more doesn't always guarantee better performance in every area. We've put each one through a series of demanding tests to show which provide the best performance for gamers, upgraders and demanding computer users. We've even tested to see which can be easily overclocked, giving you more power for free.

Next month, we'll test the latest motherboards. Together, these articles will tell you all you need to know about investing in the most important parts of your PC.

CHOOSING A... processor

If all the different kinds of chips isn't enough to confuse you, there are also considerations such as sockets, cores and clock speeds to think about. But don't clutch your head in despair - this guide will help

Many people believe that choosing the most expensive processor or the one with the highest clock speed will give the best performance. That isn't the case. Certain processors are better at specific tasks than others. Other factors, such as the number of cores and instruction sets supported, make a big difference. To complicate matters further, not all processors are compatible with all motherboards or chipsets.

SOCKET TO 'EM

As processors change, so do the ways in which they connect to the motherboard. If you're upgrading, check our 'What upgrade can I fit?' table below to see which processors will fit in your existing socket. Intel's Socket 478 and AMD's Socket A are now obsolete, so if you have one of these you'll need to add a new motherboard, too. Look out for our motherboard review in next month's Labs.

If you're adding a new processor to your old motherboard, it's important to update the motherboard's BIOS. Socket 754 motherboards usually need a BIOS update to recognise Sempron processors, and Socket 939 motherboards need updating to recognise an Athlon 64 X2. To find the latest BIOS for your motherboard, visit the manufacturer's website. If you're replacing a Pentium 4 or Celeron processor with a Pentium D, make sure your motherboard chipset supports a dual-core chip; Intel's 915 and 925 chipsets don't.

MISLEADING MEGAHERTZ

It's important to know that higher clock speeds do not necessarily equate to faster performance. AMD's Athlon 64 3800+ has a clock speed of just 2.4GHz, but our tests show that it is just as fast as a 3.2GHz Pentium 4 in most tasks. For this reason, AMD and Intel have stopped using clock speed to refer to processor models. Intel chips are given a three-digit number. The first digit indicates the processor family, with the next two indicating its speed and features. AMD uses a four-digit number to indicate the relative speed of each chip, so an Athlon 64 4000+ is faster than an Athlon 64 3200+.

A processor's performance is also dependent on which instruction sets it supports and whether it has more than one core. All AMD Athlon 64 processors and the Sempron 3400+, as well as some Intel Pentium 4 chips, have 64-bit instruction sets. These affect performance in Windows XP Professional x64 and future applications written to use 64-bit processing. Most modern processors support SSE3 instructions, which accelerate tasks such as video encoding in some software.

Dual-core processors are, in effect, two chips in one. They can almost double the speed of a PC when running applications that use both cores at once, such as our video-encoding benchmark, but they have no effect on games or other single-processor applications. Intel's Hyper-Threading allows a single-core processor to alternate between two sets of tasks easily, producing a smaller speed increase in some applications.

SOMETHING FOR NOTHING

It's not often you can get something for nothing, but certain processors allow you to get more speed for free by overclocking. This involves adjusting your computer's BIOS to run the processor at a faster clock speed. For example, you could use a processor designed to run at 2GHz and run it at the same speed as a more expensive 2.2GHz chip.

This process must be undertaken carefully, as too much speed with inadequate cooling can cook your expensive processor. If done correctly, though, you get a faster processor for no extra cost. Our reviews explain which processors can be safely overclocked. Bear in mind that you'll need a decent processor cooler and patience to get similar results.