Most of the GF cards have the overclocking utility that you can download from NVidia, or it's built into the drivers. My suggestion is that you don't mess with it. Overclocking your GPU isn't like overclocking a CPU. The video card can be alot more unstable than the processor can, and can cause hangs, freezes, or even screw up your card physically. Don't do it unless you are prepared to purchase a new card, because there's a chance you may need to.
My opinion is not to overclock anything if you are not prepared to replace it. Overclocking, IMO, is best suited for extending the life of an otherwise, outdated or borderline piece fo hardware. Our server is running on dual overclocked celerons. We were able to get a 50% increase in performance out of processors that would have been too slow otherwise.
But, its painfully obvious you have no idea what you're doing (no offense) so do yourself a favor and do research first. Figure out what you're doing, or you'll be replacing your hardware. And, NO, asking us is not research!!
IMO, the gains you actually see in games by overclocking your vid card are minimal at best...