VR-Zone has posted a guide on voltage modding the NVIDIA GeForce 6800 GS graphics card.
It is assumed that modders have a basic knowledge of Voltage modifications. For a beginner's walk through, you can check this guide.
For GPU Core voltage mod, please solder a 10K ohms variable resistor to the point marked above. Remember to set it at maximum of 10K ohms and connect the other leg of the VR to ground. Measure the voltage at the check point marked with your multimeter. Tune down resistance slowly to increase core voltage..