Usually overclocker overclock their video card using the following scenario : first overclock limit is archived with the regular cooling system, then they adapt a voltmod.

Vgpu 1.1V

Vgpu 1.15V

Vgpu 1.2V

Vgpu 1.25V

Vgpu 1.3V

Vgpu 1.35V

Vgpu 1.4V

Vgpu 1.45V

Vmem

Associate reported about voltmod for core and memory using the video card geForce 9600 GT standard design. The default supply core voltage in this video card ( in the rest state ) is equal to 1.1 v, under load it rises to 1.15 v. Increase in vcore voltage over 1.4 v, do not improve the clock frequencies.
The memory voltage supply for the geForce 9600 GT is equal to 2.0 v, with hardware modifications it is possible to bump it up to 2.1-2.2 v without special risk. In this case the memory and video chip must be adequately cooled.