If all you play is WoW, then you'd probably be fine with a pair of Nvidia GTX 260 cards (non-SLI mode, one per monitor) or AMD/ATI 4850/4870 cards (same setup). All these cards are good performance for the price range.

If you play more demanding graphical games, you may want to consider Nvidia GTX 280 cards or wait a bit for the AMD/ATI 4870x2.