Your question should be Vista or Windows 7. Do you have XP apps (not games) that you need to be able to use? If so, windows 7 has virtual XP mode built in where Vista doesnt.
Forget XP. No support for XP in the near future and going Windows 7 will pave the way for direct x 11 capabilites for your gaming needs in the future provide you get a direct x 11 video card as well.