For any gaming rig regardless of what you're going to play, the main deciding factor is how much RAM you've got. If you have more than 3GB of RAM then go with Vista 64bit, as it can recognize more than 3.4GB which is the limit for non-64bit systems. If on the other hand your RAM is lower than that, there's not much of a reason to upgrade to a 64bit OS. In that case, i would stick with the tried and proven XP until windows7 comes along.