The best card is HD5970! But if you tell us your system specs we can help you find a suitable card for your system that does not cost $600-700
I dunno, 3 way GTX 480 SLI love would probably be better than a single HD5970.
In all sincerity though, to the OP's question, an ATI 5750 would probably play WoW very well. But you might not even need that depending on what resolution you play at, and what graphical settings you wish to have.
Being single threaded, and with all the addons/plugins for WoW it can tend to be CPU bound.