At worst, get a 6950 and see if it OC's well.
Several benchmarks have shown 6950 with a +20% overdrive can perform as well or better than a stock 6970 w/o OD enabled.
I was able to mod my 6950 to 1536, but I'm using the 6950 bios as to keep the stock memory voltages. Running 890/1385 @ 1.08v. I was running 910/1400 @ 1.085v, but I was getting micro-stuttering from the VRM limiting itself. Not worth the extra OC if the card is limited by the power draw.
P.S. I do mean 1.08v, I'm under-volting the chip to 1) run cooler 2) lower power draw to allow more OC. It is running stable.