What flavor of Linux would you run on it? Mainly I'm looking for someone with experience running Linux on this machine but opinions are welcome as well.
I built a simlilar P3-600 last year to do the same sort of thing. Just wanted to point out the obvious, because it didn't hit me until I'd had make and gcc running for a couple of days straight: Don't use Gentoo.
I ended up using Ubuntu Server, but there's not much different between it and Debian. In some cases, Debian actually is more up-to-date than Ubuntu with regard to some server-side applications, like the IlohaMail PHP webmail client package.
Apparently still is, 8 months later: https://bugs.launchpad.net/ubuntu/+source/ilohamail/+bug/52462
I'd just say go with what you're comfortable with, unless it's mostly compiled from source.
-Sean Crago