I can build an Athlon 64 server for a mere $1000, if I use Ubuntu Linux as the OS. Holy crap! We budgeted $9000 for a new server (another $3000 for a tape drive). I wonder if I could get 9 little servers instead....
I don't know enough about Linux security to feel comfortable exposing one to the Intarweb just yet, so for the web/mail server, I'll probably just revamp the existing Windows 2000 machine.