You may want to take a look at Max OS X as well. I've made several attempts at running Linux as my primary desktop OS and always went back to Windows due to lack of software (MS Office, and multimedia primarily) and the constant need to futz with it to get it to do what I wanted. I bought a Mac Mini after it came out and haven't looked back since. Things generally "just work" and work well. There's no question though that the Mac route is more expensive.
Most of my experience is with Red Hat but I've installed and tested Ubuntu several times and it looks to be a competent desktop OS. SUSE is also a safe bet.
