Hello, do you know the power supply that pc has? (I've read around a little bit and if it's around 235-280w then you have a very limited choice)
The only choices would be the lower end Radeon 6450/5450 entry level cards
Since it's a small form factor PC, the PSU would probably be a custom sized OEM one which would be a pain to replace
All in all, it proably wouldn't be running WoW too good either.