I have only been able to unlock it to 12/6, 16/6 gives me artifacts. I have been able to hit 420/1160. I have an Arctic NV Silencer on the card, and my temp are very good about 47c idle 62c load. I tried the volt mod in the bios, it didn't help(I may have done it wrong).
BTW: My specs are:
P4 3.0E(Prescott)@3.6 240fsb(Which I can go up to 3.75 250fsb Prime stable, but my video card doesn't like it that high for some reason)
Asus P4P800-E Deluxe
1.5gb dual-channel Crucial Ballistix PC4000
eVGA 6800GS CO AGP
Antec TrueBlue 480w
PSU