nice setup, go for 2003. The min. specs it will run at are 133mhz processor and 128 megs RAM, so you'll be fine. You'll get more out of 2003 than you will 2000 anyways.
for what you're doing, there's no real difference you'll notice. However, server 2K3 does have better security measures built-in, and SP1 for server was released last spring, so more security features were added with that.
But then again, security is a matter of how you set your machine up, you can leave it wide open, probably making everything run with less of a hassle, or crack down on security, and work a bit harder to get your stuff up and running.
Find a security thread on here, or get a book on securing win server for more information on doing that.
I ran win2k3 on my PowerEdge 2200 in my signature. Ran all my FTP server, fileserver, router, firewall services fine with just a little bit of a wait to start some windows, but other than that not too bad at all. Similar to XP with all themes and shit turned off on a 233mhz with 256mb RAM.