I run a G4 450 at home as a server (headless). I found a cheap supplier of PCi-X 4 channel sata cards and have one of these with a couple of 350gb SATA HDs.
I run sharepoint, itunes and connect 360 and it serves all of the media in my house. Currently it runs Tiger but am upgrading it to Leopard so that I can use it for time machine for my MBP.
Currently it serves my ATV in my living room, XBOX 360 in the bedroom my MBP and an Airport Express in my conservatory and it happily streams movies and music to all 4 at once.
I know that there are a lot of Drobo fans in these forums but I recon my G4 performs easily as well and is more flexible. Yes it is bigger but I have the ability to run it as an FTP server, an apache server, DNS server, it can act as a firewall and as a time capsule, a VPN gateway and that is without running Apple Server.
It will also run Kerio as full blown mail server and with a copy of Retrospect it can backup all of your network machines too.
Dont write that little blue machine off, as a server it is fantastic.