Depends if you wanne have physx, if not i would take the 6950. PSU ; at least 600W, if you wanne sli later 800 or more. Single stable rail with at least 40A ( more if sli ). 80+ certified. Antec, Corsair, Seasonic.
I agree with these comments. The scarcity of the HD6950 has driven up prices and they are difficult to find. So you're looking at $240 with a rebate vs close to $300. I'd go with the GTX560ti, but I am an AMD fan so I just hope there are a few 6950's around when I can afford it.