Overclocking Video Cards is done with software.
some Vcard manufacturers supply tweaking software with their bundle, but I think many (most?) users
of NVidia cards prefer to use NVidia's reference drivers (detonatorXP). These drivers do not automatically
enable overclocking features. There are several software utilities available that allow/enable overclocking
of your video card.
For what it's worth, I prefer
RivaTuner
PS: the
Guru3d is a great site for learning (there are many others too)