XP Pro, definitely. It has MP support, so if you have a recent core duo-based Mac, this will be a plus - even though it is only available in a handful of games, it can make a difference.
XP Pro x64 is based on the Windows Server 2003 SP1 x64 code base, and is very stable. But I wouldn't recommend it for games because of driver issues. Since it wasn't widely adopted, not all hardware manufacturers released drivers for it.