Linux: Because it's free LOL, no seriously it's free that's why I would prefer it, but if you don't know the inside out of Linux, you'll have a hard time with it at first but you'll learn.
What is FreeBSD? No, FreeBSD is a very reliable and robust OS, I have never used it but I have heard good news.
Windows 2003: It's pretty easy to setup, but why would you pay $500-$800 for this when you can do the same in Linux. Know what I'm saying? I have it running on my system, don't ask me why LOL. Just to play around with it I guess.
Novell NetWare?: Don't go this route unless you like it, because they will stop support for NetWare in 2010 I beleieve, since Novell is moving towards SUSE now and focusing all their resource on it.
IBM, HP: Don't know anything about them.