Regarding O/S's, just want to emphasize that Applications should determine which O/S you use. Choosing Vista, XP, Linux, Leopard, UNIX or whatever should only be considered after you know what you want to run. For me, I use WinXP because the applications I want to run are only written for WinXP so the choice is easy. Doesn't matter how good Linux is or how few viruses are on Mac's because I can't run my apps on them.
Games? there is really only one choice (WinXP)
Business/Financial software? (WinXP)
Music and Media Production? Either but I'd choose Apple OS
Server? UNIX
Linux? Well, you can pretty much do everything on Linux except run the mainstream applications which may be problematic depending on your needs.
There is NO best O/S, sorry there is simply not. All have their purpose.