The gang at HardwareSecrets have posted a monstrous guide that will most definitely get you going on oc'ing your video card. They cover every aspect of the process, from after-market cooling to BIOS flashing in a total of twelve pages.

"You can increase the gaming performance of your computer by overclocking your video card. Overclocking is a technique that makes a given hardware part to operate at a clock frequency above its standard frequency, thus increasing its performance."