I have both a G4 and a G5. The G5 runs as a desktop in the study and is mainly used for word processing and web browsing. Nice machine and, as others have said, takes 2 SATA drives straight out of the box. Also runs Leopard, which may end up being important to you - more of which once I explain the server.
The G4 (AGP 500) runs as the server. It has gigabit ethernet which means it's up with the rest of my network. Boots off a newish 120GB Seagate IDE drive and has a Sonnet PCI SATA card with 2 ports; each port has a SATA drive attached - if you're really keen, the G4s can take 5 hard drives; there is literally nowhere to put more than 2 in the G4. It has 2GB of RAM, 2TB SATA storage and quite happily serves multiple iTunes clients (ATVs and laptops). Runs like a dream, no performance issues, even off a 10 year old 500Mhz machine.
I built the server on my desk (hard drive upgrades, RAM etc) and run it headless in the attic where it's nice and cold - it runs 24x7.
Now, all went fine until this Christmas when we got my son a new Apple TV for his bedroom. Wouldn't connect or even see the server. Eventually I worked out the following:
- ATV 2 can only stream from iTunes 10
- iTunes 10 can only run on Leopard or above
- the server was running Tiger
- and theoretically couldn't be upgraded to Leopard as Apple imposed a minimum performance standard of 867MHz
So, bit of Googling (and indeed hunting around here) later and I have managed to upgrade it to Leopard and all's well again. But that's something you have to be aware of if you go G4. Also, frankly, they're old machines. If you want to run it as a server, do that. But don't plan on humping the thing around, doing lots of upgrading, multiple power cycles a day etc etc - just set it up, hit go and let it do its thing.
If you go G5, it's a newer machine. It can run Leopard straight out of the box and it looks great. However, it's also bloody loud - you really, really don't want one of these in your lounge. In addition, I seem to remember that the power consumption is a fair bit higher on the G5, which is a bit pointless if you're only using it as a server - particularly if it's one of the faster dual processor jobs. And, as others have said, it'll only take 2 SATA drives.
Ah, it's that time of night - "there is literally nowhere to put more than 2 in the G4" should have read:
"there's literally nowhere to put more than 2 drives in MY G5"
...later ones did have more drive bays.
Apologies.