I play GW on my Mac. Since you asked – my Mac is a Mac Pro that I use mostly for graphic design work, and has two 3ghz dual-core cpus, 4gb memory, and I run OSX 10.5 side by side Vista w/ boot camp.
I am sure you'll like Windows XP more than Vista. I have strange internet problems on Vista that I never had with Windows XP and I don't have with OSX. I used to be all about Windows in general, but now I prefer working with Mac OSX. The only thing I would like to see is a more up to date selection of graphics cards, although the 7300GT that I have in this Mac Pro is decent.
May I make these buying suggestions:
• WinXP instead of Vista
• Get a more normal mouse if at least for gaming.
I'd highly recommend getting a Mac, because I know you'll be happy with it.