Windows Ultimate has both 32bit and 64bit and windows 64bit alows you to use more than 4gb of ram up to whatever so that if you wanted to use more than 4gb of ram then you would get the 64bit version. I think that the 64bit is the OS you want to use for gaming and you can get either Ultimate or Professional would be good choices and you can get lower priced versions of 64bit.
just get home premium... you don't need professional unless you want to join your system to a domain and you don't need ultimate unless you want the OS to support more than 16GB of RAM.... in which case go with ultimate...
Windows XP was without a doudt the best OS that Microsoft has ever put out and it was a sad day when Microsoft said that they will not longer be supporting XP , that means no more updates , no more patches , no more service packs and you will be hard pressed to find it for sale somewhere. Internet Explorer no longer supports XP so you have to make do with earlier versions. I used XP for years and never went to Vista but have now gone with Win 7 and I do think it is every bit as good as XP. Just like Internet Explorer I am using IE8 and have tried IE9 but don't like it at all and want to stay with 8.
Why the bleep would you want to game on backtrack??????? You give linux a bad name. Thats a hackers OS that has hacker tools everywhere, you can gain entrance to wifi, passwords, pretty much anything you want with backtrack. Thats why we used it in my ethical hacking class for that reason.
The only linux OS i would even consider for gaming use would be ubuntu with wine.
You can get w7 pro for $65 if you know someone with a student email. Pro also support 16gb+ like ultimate. There's really no reason for ultimate as the extra features are mainly encryption which sucks and you can get free encyrption that is better. All of them game the same, I doubt games will ever use 16gb ram before you pc is obsolete. Get home for the cheapest option, pro for 16gb+, domain join and xp mode, and ultimate if you have money burning a hole in your pocket. http://en.wikipedia.org/wiki/Windows_7_editions