I just found out my voltmod guide is the first hit on a google search:
I can confirm that this definitely does work on 256MB 7950 GT's.
I put my 7900 GT into a friends computer (he has an X2 3800+ at 2.4GHZ) and it shorted out - I am pretty sure it was his power supply that did it.
So I went and got a Leadtek 256MB 7950 GT (AU$355, which is a great price), and used my conductive pen to add 0.2V to it (stock voltage is 1.3V in 2D, 1.4V in 3D).
at the current voltage it works at 710MHZ. I'll try upping the voltage later to try and get past 750 again.