Right now it's hard to choose. Each choice has advantages and disadvantages. 64 bit Vista is the future. 32 bit XP is the past. If you want to optimize the box for five copies of WoW, the past probably still works slightly better (WoW is a 32 bit program so it probably runs very slightly faster on a 32 bit operating system). But the pc can't use more than approximately 3.5 gigs of ram with a 32 bit operating system and within the lifetime of the pc, you'll almost certainly want to add more. So if you don't get 64 bit Vista now, you'll probably switch over to it in a couple of years.