Search for "coolbits" its a .reg file. You run it, it changes a couple settings for the nvidia control panel in the registry unlocking the overclocking features.
Check out http://www.pcstats.com/articleview.cfm?articleID=1803
it goes into overclocking with both the Nvidia Control panel and Riva Tuner.
One way to tell if the card is running too hot is you will increase the mhz speed. Benchmark without any artifacts but the bench mark score goes down. If this happens the video card is running too hot and throttles itself down.