IMO it depends on the hardware and software you're going to use in addition. WINE has gotten VERY good at supporting WoW and I played my alt when I 2 boxed under Linux for over a year with no issues. When I went to 5 though (on 2 machines) I switched to XP because there are no (to my knowledge) drivers for the XKeys Pro USB which I use. Now if you're going to use something like the Nostromo n52 there is an open source, although pretty much dead, project that worked well for me. On the software side apps like Keyclone and Octopus are Windows only. Also if you use the PS/2 Xkeys and a Vetra then Linux definitely won't be an issue.

I definitely feel your pain. 5x Windows licenses, oem even, would be $750!!! I didn't spend that much on my quad core machine. If your hardware/software dictates you have to go windows then at this point I'd suggest looking into MSDN OS subscription for $700, but I'm not sure of the recurring cost to keep it going.

Good luck and let us know how it goes. I love to here WoW linux success stories. Not because I'm an MS hater, I don't really care, I just look at it from a cost perspective.