I've been running Windows XP on my main computer for many many years. I've been playing wow for over 3 years now and will say that they ran together well enough.

Recently I got my hands on the Windows 7's final release build. I've been using it for over a month now, and wow has never worked better and smoother with Windows 7. In addition, I specifically installed the 64 bit version of Windows 7 on my PC since I have AMD 64 Athlon ( finally taking advantage of the 64 bit architecture). Through all of this, wow works great, just as smooth if not smoother as it was under Windows XP.

Just remember - you don't have to reinstall wow when you install Windows 7. Just back the whole wow folder to another drive, then just copy it back and run wow - no installation hassle.