If you are talking about a dual card setup(SLI/Crossfire) the HD6950 2GB is the way to go in that price range IMO. The extra memory helps get the most out of such a large amount of processing power, especially at high resolutions/on triple monitor setups.
Yes i am talking about sli/crossfire set up but will only be on a single monitor. This is for a new build i am thinking about. Are you saying dual HD6950s 2gb or just 1 off them in place of dual cards?
Well, if your monitor is standard HD(1080p) then just one HD6950 should do very well(as would just one of the other cards you mention.) I would just get one then decide when/if you need another. If you do end up needing/wanting two cards then it is best to have the 2GB on the HD6950 IMO.