I'm running a 6600GT and use the following to overclock click here When over clocking keep a very close eye on core temps do a little at a time and take note of totally-braindead... there is risk involved, although i've had no problems of yet!
Cool bits is good bit of software for overclocking, however, watch those temps! upgrade fans etc etc, perhaps watercolling if you really get hooked! Have a look at Custom PC's overclocking guide in their forums, and if your feeling brave then have a go!! click here
however, IT CAN BREAK YOUR COMPUTER IRREVERSIBLY!! USE CAUTION!!
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