Clearly the best way is to go 6950 2GB and unlock it.
However i dont get people recommending to get 560 because it can overclock to 1ghz.
6950 can do that too people.First reviews werent getting there because there was no software.
6950 2GB has more power,consumes LESS power, overclocks the same, and can be unlocked to 6970 while costing like 20$ more.For me its a no brainer.
GTX 560 will be good too, but 1GB is starting to become bottleneck with high enough settings.And theres nothing to unlock.