First download this file: geforce_overclock.reg
(Easily found using google)
Double Click The File
Click YES
Click OK
Now Right Click The Desktop And Go Into Properties
Select The Settings TAB
Click Advanced... Button
Select The TAB With Your Card Name On It
Click The Clock Frequencies Option At The Side
I suggest slowly moving the bars up maybe 5MHz at a time. Clocking the memory normally gives a more noticeable change in your graphics card performance. If Your system sounds funny, stops/freezes or crashes put your settings back to normal!!
So far I've clocked my TNT2 M64 32Mb SDRAM card from 125MHz Clock speed to 145MHz and from 125MHz Memory Speed to 150Mhz without a change in system temperature or system stability