USAGE FROM MOST TO LEAST IMPORTANT: Gaming (All Valve games, BFBC2, SC2, COD: Black Ops, Mass Effect 2, Star Wars: Old Republic, Diablo 3)
CURRENT GPU AND POWER SUPPLY: GPU - 1 GB ATI Radeon 4650 PSU - Corsair TX 750W
OTHER RELEVANT SYSTEM SPECS:
Core i7 860
ASUS P7P55D-E LX LGA 1156 (no SLI or CF)
G.skill Ripjaws 8 GB RAM
Coolermaster HAF 922
PREFERRED WEBSITE(S) FOR PARTS: No preference, but probably Newegg
PARTS PREFERENCES: AMD 6970 or nVidia GTX 570
SLI OR CROSSFIRE: No (at least not with current mobo)
MONITOR RESOLUTION: 2048x1152
I have created another thread about whether or not to get a 6970 or 570. I am currently leaning towards getting the 6970 for the extra memory and what appears to be a slight advantage at higher resolutions. Otherwise it seems to be a dead heat. The difference in price doesn't make that big of a difference for me. It is just a matter of what is the better card. I am planning on using this card for quite some time.
The 6970 will serve you better at the very high rezolution you are running with its 2gb of memory. They are about the same speed otherwise. The 6970 may be faster in the long run, but there is no assurance of that.
In my research, I see the performance in the games you listed as basically similar. The main difference I saw was that GTX 570 has a little bit better heatsink than the HD6970 that keeps the card quieter and cooler. That will be even more apparent with a modded 6950 to 6970.
Or you can go for the GTX570, which has a stock vapor chamber cooler that was used as custom cooling on several last generation cards. Custom coolers will vent heat inside the case. On a GTX460, they increase case temperatures by ~4c. On a much hotter 6970, which pushes operating temperatures towards the 90c range, the impact on case temperatures will be much greater than 4c. Custom coolers trade off lower GPU temps for higher case temps.
I would either unlock an HD6950 or go for the GTX 570. At its current price the HD6970 doesn't really make much sense.
That has been the lesson learned this week with the BIOS unlocking revelation. If you are comfortable with flashing your BIOS, there is little reason to get a 6970 (unless you want the 6970's higher rated memory). Despite the dual BIOS recovery option, I'm wondering if there are any hidden drawbacks that we are yet to see in the modded 6950>6970's.