well if your card doesn't have to outputs (VGA / DVI) then you're going to have to get a new one, but for the time being you should be able to get away with your card. Worst case you may have low FPS. But for leveling high FPs really isn't that big of a deal. Just go make dinner and watch a movie while loading ogrimmar.

And as far as AMD's expansion I hadn't heard anything about that. I just know that I've never had any problems with AMD.