Unless you're running 64bit versions of your software, there's no need to use a 64bit OS, IMO. The best processor right now is an Intel Core2 Duo. Blows away Athlon X2's for speed, and runs a lot cooler at the same time. I have an X2 4400, and I like it, but if buying now I'd get the Core2 Duo. They weren't out when I built my current machine. I'd go with XP Pro, btw.