Need advice for custom computer?

I am trying to decide if I want Windows XP with 4 GB RAM, or if I should get Windows Vista with 8 GB. Also where could I find a copy of Windows XP?





I already have experience with Windows Vista, however my laptop that ran it only had 512 MB RAM, so I switched it to Ubuntu, but now I want a fast gaming computer.Need advice for custom computer?
Vista with 8 GB RAM and a new Core Duo Extreme will be perfect!