I have never play wow and quite frankly know very little about it's specs but looking at the website for recommended video cards, and just from the fact that i know wow isn't that demanding, an 5850 is complete overkill for wow by a long shot, so would 4870's and 4890's in crossfire, one 4870 would probably be able to run this game a well over 100 fps
WoW used to not be demanding, but since the last expansion, it is very demanding, at least with high settings. A single 5870 with an i7 920, at 1920x1200 struggles a lot at ultra settings with shadows turned down a couple notches.
Granted, you can turn the settings down some, and it would run smooth, but the point is, a 5850 can be brought to it's knees with WoW. It's not overkill, unless you are fine with lower settings.