go for gtx570 or hd6950, with 6950, you may unlock to 6970. best value
As Vind said however.
Be careful with the 6950's as you will struggle to find an early model which could be unlocked. Most of the new revision models of all 6950's will no longer unlock once flashed.
However i would go with a 6950, currently bought the HIS iceq turbo 6970 which is great.