XP Pro 32bit if you are getting a Mac Pro, otherwise it doesn't matter.
XP Home only has multicore processor support so it will be good with any Mac that is not a Mac Pro but will only see one processor in multi processor systems such as the Mac Pro, XP Pro has multi processor support.
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.
You guys are misinterpreting what multiprocessor support means (and it shouldn't be too hard considering its literally what it means).
Dual core is NOT dual processor, its just a single processor, XP Home DOES support multicore processors so any Core Duo (and all of its cores) will be utilized to the fullest of its performance in XP Home.
On multi processor macs like the Mac Pro (the ONLY Intel Mac with multi processor) XP Home will only see and use ONE processor and all 2/4 cores on that processor.
So yeah, the point still stands, XP Pro 32 bit is your best choice but if you get Home your performance will still be the same if you don't have a Mac Pro. Don't get 64-bit and thats pretty much all you need to know.