The ultimate guide to overclocking

Around this time, RAM overclocking became more common place, as memory speeds were ratified, and with that came more tweaking of the front-side bus to compensate for the locked multipliers. Overclocking shifted further towards the BIOS and away from jumpers, which in turn led to overclocking software.

The first was 1998's SoftFSB, which enabled bus-tweaking from within Windows for the first time. With the Pentium III era came aftermarket coolers, as processors now chucked out so much heat that a standard cooling block and fan wasn't enough to cope with an overclocked chip. And so it continued, overclocking largely becoming easier and more commonplace with each processor generation.

This leads us to the Core 2 chips of today, and Intel's current terrifyingly unassailable dominance of the CPU market. Generally drawing as little as half the power of the Pentium 4s that preceded them, most of the range offers a vast amount of overclocking headroom, to the point that a low-end Core 2 Duo can almost go toe-to-toe with the top of the line.

How to overclock

So how's it done? Key to processor overclocking is the front side bus (FSB). In the very simplest terms, this is the connection between the CPU and the rest of the PC, and its speed defines the processor's speed to a significant extent. Intel CPUs' final speed is the FSB times the multiplier - so if you've got an FSB of 266MHz and a multipler of 9, your chip will run at approximately 2.4GHz.

While the multiplier is usually locked – though some chips let you at least lower it, to conserve power and reduce heat – the FSB isn't. Bump up the FSB and you bump up the chip. In our example taking the bus to 290MHz gives us a 2.6GHz processor. This is no random example, incidentally, it's what we run the Intel Core 2 Quad Q6600 in one of our office test systems at, giving it a healthy 200MHz boost that makes a noticeable difference in CPU-intesive games and hi-def video re-encodes.

What stops us from going higher? Not a lot in the case of this particular chip. We're playing it safe for desktop work, cos we're in a particularly sweaty office. When we're farting around with high-end tasks, we can have it running stably at over 3.3GHz (with an FSB of 370 or so) on a decentish, third-party air cooler.

That's more or less trading blows with the best Intel has to offer on a £120 chip. But while going to 280MHz on the FSB took a BIOS tweak, a reboot and Microsoft BOB's your uncle, going much higher does involve more fuss. First up, when our Q6600 is at 3.3GHz, it's also running at nearly 70ºC when under maximum load (and around 50ºC when idling).

It's perfectly stable, but it could damage it in the long run, and on top of that the fan is making enough noise to wake the deaf pensioner in the next street over. Watercooling, a fancier air- cooler or even just a spot of dust- cleaning will bring the heat down, but there can come a point where that stuff becomes more expensive and hassle than simply buying a better processor.

The second hurdle is the motherboard. Pushing up the FSB doesn't affect only the CPU, but also the mobo and, in many cases, the RAM and PCI-e slot to boot. In our case, we're using a motherboard that supports a monstrously high FSB. When shopping for a mobo, its max FSB will usually be referred to as four times the actual speed, due to the way the processor actually fetches data. So when we've got the FSB set to 266MHz, in effect that's 1,066MHz.