WoW doesn't take elite horsepower to play. Back in the day, i ran it on a P4 3.0 Prescott with 2gb RAM, a 7800GS, on high settings at 1280x1024 and it ran fluid smooth all day long. The game isn't very CPU intense. I recall sitting in Shattrath, with all the fog effects and such, still getting 60+ FPS, on my P4 3.0/7800GS setup. A 5670 would run the crap out of that game.
Toms did a test
with the release of Catacombs that seems to contradict you.
As to OP, tests have shown that WoW runs best on nVidia cards, and the more VRAM the better, that said, the Radeon 5670 being suggested would suit your current setup fine.
Edit: heh and /thread as I was looking for the review,