MSI Afterburner may work but it is usually recommended (from what I've read) for ATi cards. If it doesn't work then try RivaTuner which is primarily for nVidia cards (can do basic clock settings on ATi too). Your card has to have a voltage controller capable of setting the voltage by hand, many cards do not have a compatible controller and cannot be voltage modded without hardware hacking.
The only other way would be the nVidia BIOS (not the motherboard BIOS) which may allow you to set a permanent overclock, probably not a voltage change though.