Im not making it AMD v Intel as there is no comparison, Intel wins every time.
However, to make a blanket statement that "Than your going to be stuck with sub-par performance because WoW hates AMD chips." is total crap.
Sure AMD's dont have the same performance, but they don't hate the game. Hating the game would mean that it would not work at all, or that its Performance hinders the game to such a point that it is unplayable.
Also, It would actually help to know what make and model the Mainboard is so we can get a better idea of how big of an upgrade you can actually do.