Since Skyrim is due out in the fall you will have a new crop of video cards to consider.
From past history, Bethesda games are usually no more than a generation behind in their requirements so that makes one of the higher 5XXX series the the best choice but it's still speculation.
Go over to the Rage3D site :
Rage3D Discussion Area - Powered by vBulletin and check out how the various crossfire set-ups are working out before you commit.