Free software – some of it official NVIDIA/ATI driver plug-ins – will do the trick from within Windows, and built-in safety cut-offs and stability tests make it incredibly hard to damage the card, though of course you are going beyond the warranty. It's also grown a little more complicated of late in that you may need to overclock the shader clock as well as the GPU and RAM for the best boosts.
In the case of NVIDIA cards, it used to be that this was twinned to the GPU speed, meaning a raise in one had a synchronous effect on the other, but for a little while now they've been able to be altered independently. So if you hit the speed ceiling on the GPU, it may yet be possible to eke more performance out of the card by pushing the shader clock a little further.
While the present situation is that you can overclock everything and be pretty confident it'll work, the future of the form is harder to call. One thing seems sure: it's not a dirty little nerdy secret anymore, but an increasingly common practice, most especially with Core 2 chips.
There's a vast aftermarket cooler industry to support it, and even cheap mobos can handle a bit of a free boost. If anything overclocking will become easier, with more and better applications to achieve it within Windows, rather than from the BIOS, and possibly more in the way of automatic volt-modding. But much depends on the future of desktop processing.
There's a big war brewing between Intel and NVIDIA as to whether the CPU or the GPU will be the major element in the PC of the near- future. Intel are pushing ray-tracing, using a multi-core CPU to render game graphics, while NVIDIA's CUDA enables its recent GeForce cards to perform parallel processing, such as video encoding and in-game physics, far faster than a CPU could manage. If either of these bed in, overclocking will need to take them into account.
At the same time, the slow move to ever-more cores potentially reduces the need for conventional overclocking, as raw clock speed continues to be a lesser concern to multi-threading and, in the case of 3D cards, the number of stream processors and texture units. That's hardly going to stop anyone from trying it, of course.
Even when its effects are minimal, overclocking's always going to be a sure-fire way of making a system feel like its yours rather than simply a collection of mass-produced parts. Modding the case is one thing, but what makes a PC is its performance. When you've painstakingly tweaked that performance into something that suits your own purposes, and it's become something that feels like you've gone far beyond what you paid for it, the system will feel more unique than all the green neon tubing in the world could ever hope to achieve.
While some overclocking can be done from within Windows (some mobo makers provide an app, while NVIDIA's own nTune's your man if you have an nForce board. Systool, from techpowerup.com/systool, may do the trick if you're still running Windows XP) most likely you'll be doing this from the BIOS.
Lingo varies hugely from BIOS to BIOS, so you'll need to have a scour for the screen dealing with CPUs and FSBs. It could be called 'Advanced' or possibly 'CPU settings'. On our test rig it's described as 'jumper-free configuration'. Mobo makers really don't make this easy. If you're running an unlocked chip such as the Intel Extreme Edition or AMD Phenom Black, you'll be able to alter the option called 'multiplier.' This will raise the clockspeed and nothing else in the system.