Head up, this is all coming from a newb overclocker. You can use google as an alternative.
Well you can use the original overclocking app available at http://www.nvidia.com/object/nvidia_system_tools_6.02.html. Just make sure you monitor the temperature. and it doesn't get too hot. Or you can use Riva Tuner as an alternative.
To overclock it you just have to find the settings for your gpu when you are using one of the apps and edit them, preferrably make them higher. J
ust make sure that the Core Clock and the Memory Clock does not go higher than the Shader Clock. And make sure that the Memory Clock does not go higher than the Core Clock.
The only problem with overclocking a gpu like that is that it will frequently crash a lot more than usual. I've had first had experience with my 8600GT with it black screening if things get too hectic.
BTW here's a ss of my overclock app.