Also depends what you're planning on "serving". My G4 500 runs Tiger and handles file & print sharing and iTunes to 2 laptops, a desktop and an Apple TV without requiring the server version. I can't remember what the user limit is (5? 10?) but you may well find plain old Tiger works fine.
The big issue I've had to deal with on my server is the lack of support for big disks. I finished up with an add on board to give me SATA compatibility but depends whether you want to spend the cash.
The other advantage of the G4 over the iMac is that it's a doddle to work on and takes more drives - the G4 500 takes 4; I'm assuming yours (which is newer) will too.