Thank you for the reply. I didn't think to take into consideration the RAM; I would be looking at the 2GB 6950.
I do experience a lot of issue at 1600 with the 4870s and I am happy just to get them to my native resolution in games like Fallout 3. I find a lot of time it's best to run the 4870s at 1920x1080.
If a single 6950 HD 2GB will run 2560x1600 I would be happy, but I have not been able to find any proof as to what kind of FPS I can expect.