Now you reboot, and then right click on your desktop, go to Properties, then Settings, then Advanced, then the 6600GT tab, and finally the "Clock settings" tab on the pop out tab.
Be sure you only increase about 10 MHz at a time, and run a benchmarking program or game (I used 3DMark03) and watch for any artifacting (textures being drawn incorrectly). If you get artifacting, lower the clock a little bit and try it again.