Follow this and it'll tell you how to enable coolbits. Not sure if value 3 will work for you. In some configurations you'll have to enter 1a - search the web for info on coolbits values - I didn't bother.
When enabled, you'll get an overclock menu in your driver. If using a recent driver, go to start - settings - control panel - nvidia control panel - 3d options - and you should have overclocking.
I'm sitting on a hp workstation with some silly onboard chip right now, so perhaps the names aren't 100% - but it can't really go too wrong
Edit : By the way - if only one slider (memory or core) is moved, the test button sometimes doesn't work. So move both sliders every time you want to test.
Alternatively take a look at firingsquads forums - I saw a thread on overclocking benchmarks where they mentioned serveral good tools for overclocking nvidia cards.