Let’s take a look at the 7900GT next:

79gtvmod  NVIDIA 7900GTX, 7900GT & 7600GT Voltage Mods

With the back of the card facing you, look to the left of the
card and you will see the area pictured above.

You can measure GPU Voltage and Memory Voltage from the points
marked above with your multimeter. GPU Voltage runs at 1.2v in 2D mode and
keeps at 1.2v in 3D, while Memory voltage remains at 2.02v constant.

For VMem increase, look for the resistor circled and labled
VMem. Original resistance is 608 ohms. Use a sharp pencil and shade the resistor
to decrease it’s resistance. Dropping the resistance to 553 ohms from 608 ohms
increases voltage from 2.02v to 2.23v. Alternatively, you can solder a 10K ohms
variable resistor across the resistor, set at maximum resistance of 10K ohms.
Lower resistance to increase voltage.

For VGPU increase, solder a 50K ohms variable resistor onto
one end of the resistor shown above. Set at maximum resistance of 50K ohms.
Lower resistance to increase voltage.

UPDATE:

For the VGPU Mod, a better method which allows more voltage
has been discovered by the wisemen from the XtremeSystems
forums.
You can solely do the following without doing my previous VGPU mod.

79mmod2  NVIDIA 7900GTX, 7900GT & 7600GT Voltage Mods

First remove the resistor marked above and replace it with
a 50K ohms resistor. This will give you 1.5v for VGPU.

If more voltage is desired, I suggest you move onto step 2:
removing the 2 resistors marked to disable overcurrent protection for the card.

You can then move onto step 3: Soldering and connecting this
point to any grounded spot. This will give you 1.6v.

Instead of step3, you can do step 4 if 1.7v is required.