Not every model of videocard will overclock. Some won't OC at all. My old GeForce 4 TI4200 wouldn't OC 1 MHz without freezing up. I'd do extensive research on your videocard before trying to OC it.
If you do want to OC it, I recommend using Coolbits, a program hidden within NVIDIA's drivers. Do a Google search to figure out how to unlock and use it.
-Lasereth