Well, when u overclock a vidcard, u use a program. U can get a good one here:
http://www.tweakfiles.com/video/geforcetweakutility.html
When u have installed the program, go into overclocking-part of it. There are 2 squares one is memory speed, the other is core speed. Slowly increase these two numbers, one at the time, I think u should start with the memory. When u have increased the number, run 3D mark a couple of times, and u are getting a nice clean picture, increase a little more. When u are starting to get some weird pictures ( Dot's, funny colors and lines), u go a few MHz down, and that's about it. Then u do the same with the core speed.
It's actually very simple.
Good luck