The Gainward 6600GT PCI Express card is a pretty simple card to voltage mod.
I would assume that many of the PCI Express cards can be modded the same way
after scanning the back of these cards. We start off with measuring the GPU core voltage.

 Gainward GeForce 6600 GT PCI E Overclocking & Voltage Modding

You can measure the VGPU at the pin shown above, which is located behind the
card. Simply touch the red tip of your multimeter to point and black tip of your
multimeter to any grounded spot. Default voltage is 1.24v at before loading
windows. To increase voltage, solder a 10K ohms variable resistor to each ends
of the resistor shown, which is located at the front of the card.

 Gainward GeForce 6600 GT PCI E Overclocking & Voltage Modding

The original resistance is 550 ohms. Set 10K VR at maximum resistance and
tune down to increase voltage.

You can also use a pencil to voltage mod if you’re not comfortable with
soldering. Use a 2B pencil and pencil the same resistor:

 Gainward GeForce 6600 GT PCI E Overclocking & Voltage Modding

Default resistance of the resistor is 550 ohms. You should measure this
before penciling. Pencil stroke by stroke and measure the resistance. Each drop
of 25 ohms will give you roughly 0.05v more. Reducing from 550 ohms to 480
ohms
gives me 1.4v at start up, about 0.15v extra. Of course, do remember to put on a
tape over the area so the graphite doesn’t get blown off.

We also tried this with the Winfast 6600GT PCI Express and it
works as well:

 Gainward GeForce 6600 GT PCI E Overclocking & Voltage Modding