The biggest issues you have is the power supply to upgrade the video card. Although a used higher end video card can be cheap (I got my 9800 for $40), you would want a more robust power supply. 220 watt is very marginal even for a lower end video card to ensure you don't have any odd reboots, crashes.
Look here http://www.tomshardware.com/reviews/gaming-graphics-radeon-hd-6870-radeon-hd-6850,2782.html,
even the lowest card on this list will play WoW pretty good on High setting. It "may" work fine with your current power supply, but an upgrade to that is about $50 or less. You can get a very usable system for games for about $100.
I would not expect good results from your base setup at anything over Low settings.