Keep in mind, a 5770 may play almost maxed, but you'll have to turn shadows off, or only at the lowest quality. If you don't mind that, it'll get the job done. If you want to play at the highest settings, including shadows maxed, you'll need at least a 480. If you don't mind turning shadows down 1 notch, a 5870 will work.
This if you play at 1920x1200.
The good thing is, WoW does look pretty decent even when it's not maxed, but I do find shadows at one notch below max is needed for maximum enjoyment in my experience.