The Win 7 release candidate is supposed to be generally available on May 5, and work for a year. I'd go with that.

But, between XP and Vista, if you are planning to use multiple monitors, you should go Vista. It has better native support for it, whereas with XP you would need desktop spanning software to be able to move 3D windows between them without a performance hit. Just disable Aero Other than that, not much different. I use XP64 for my multiboxing personally and have had 0 problems.