Does the $650 budget include a monitor?
If not you can get a HP p6370t desktop computer with a Core i3 530 2.93Ghz CPU, 4GB RAM, 640GB HDD and an upgrade HD 4350 512MB video card for about $600 from the HP store. Should handle WoW very nicely, along with having plenty of power for other computer tasks and all but the most graphically demanding games.
Oddly enough in my experience WOW plays just as well on a GeForce 8800 as it does on my GTX 285. So I wouldn't go overboard on the graphics card and would instead get a high GHz dual core cpu. Maybe the e8400.