This is a relatively simple mod with some soldering skills.
Parts needed:
1 10K pot (variable resistor)
Wire cutters/strippers (pocket knife works too)
2 wires cut to desired length.
1 soldering iron (35w or less recommended so as to not damage circuit board or solder pads)
rosin core solder.
Operation:
First, tin the leads of the 10k pot, and also both ends of the wires. You may also want to prep the legs of the IC to make a quick connection.
I glued the 10k pot down before performing the soldering operation, this is optional, it just made it easier. Also make sure the pot is set to it's maximum resistance before soldering and turning on the power.
Solder one wire from the 10k pot to pin 5, and solder the other wire from the 10k pot to pin 7. See picture below (click to enlarge)
After that's done (if you're skilled at soldering, it will take all of about 2-3 minutes to do the whole thing), plug the card in and turn on the machine.
Read the voltage from the two points in Illustration 2. My default voltage was 1.3v. To get my core speed to 650, the voltage is set to 1.45v. At this point I don't feel safe raising it higher without a cooling mod. To change the voltage, either power off the machine and adjust the pot, or it can be done while the machine is on....BUT IT MUST BE DONE SLOWLY! I turned the machine off to prevent any accidents...I take no responsibility if you screw up by adjusting the voltage while the card is running.
Illustration 1:
Illustration 2:
