Ok, a little research and XP Pro (not Home) can support the quad core, so long as all 4 cores are on the same CPU.
My confusion came from the days when dual CPU or quad CPU meant a server board in a desktop shell - XP could do 2 distinct CPU's, but no more. Apparently as long as the multiple cores are on the same CPU Die, you are fine.
Edit: It looks like Home will do multi-cpu's as well, so long as they are on the same CPU Die...so home or pro will handle it, but I'd upgrade to Pro if you are able too. Home cannot do any more than one CPU socket, while Pro can do two (not to mention the networking components stripped from home that Pro has).
Edit2: If you're building this for WoW framerates, go cheap. MMO's are notorious for poor performance no matter how much money you throw at them - see Verissi's post above - I couldn't have said it better. If you are looking for over-all gaming performance, then by all means go as big as you can go, but don't expect miracles in frame rate from WoW.