I had the same question a month ago when building a new PC and am very glad I decided to go with adding a second 4850 rather than spend more on a newer card. Almost any game I play in 1680x1050 gets good to very good fps on high settings, though my 4850's are 1gb each, so I'm not sure how much of an impact that could have on your performance. If the impact is minimal than I would definitely say go with another 4850.
I am running dual screens. I have a 26" at 1920*1200 and a 22" at 1680*1050. I am running SC2 at Ultra settings with no visible stutters. I am not sure if this will improve performance or if it will matter.