If your NVIDIA GeForce 6 or 7 series overclocking adventures haven't yielded the kind of results you had hoped, this guide on the AOA Forums may be of interest. They outline the steps necessary to soft-mod the BIOS on many GeForce cards to increase the GPU's voltage using NiBiTor and nvFlash. I've done this mod on a GeForce 7800 GT card with some success, but it's not going to work for everyone because some board partners already tweak their voltages to support increased clock speeds on factory overclocked cards.
Also keep in mind that anytime you increase the voltage being supplied to a component, its temperature is likely to increase. If you soft-mod your card to increase your GPU's voltage, you may want to think about some additional cooling as well.