I know a hd5670 works only off the power from the PCI-e slot. I'm not sure what available wattage you have left in there to spare though. I know a 5670 will run WoW pretty well, as I ran one for a while on a old Dell that was bottlenecked by a pentium 4 HT
I'm not sure how hard it would be to find a 5670 if you have spare power for one. Good luck though!