Specifically designed for video adapters that use the nVidia chipset, nVidia’s Detonator driver or the RivaTuner utility give you control over your video clock speeds.
The first thing you need to make your nVidia chipset overclockable is a small bit of Windows Registry hacking, which is made simple by downloading a file named geforce_overclock.reg within geforce_overclock.zip (link and content subject to change) from http://www.softpedia.com/public/cat/12/1/12-1-31.shtml. To merge the Registry entries in this file into the Registry:
Download and unzip geforce_overclock.zip.
Locate and double-click the reg file within the zip file to merge it into the Registry.