8 year old G4s can run 10.4 or 10.5. A software RAID is easy and doable using PATA drives or SATA with a card. These are only USB 1.1 and not the 400mpbs USB 2.0. Again, a card is needed. Memory tops out at 1.5-2 Gig. Some have the 137 Gig hard drive limit issue. CPU upgrades are possible that add a second processor and more than triple the speed. These boxes are plentiful and cheap at $100+ depending on the model and features installed.
Links for how to use an Old Power Mac G4 class as a network file server.
This is a quick, terse walkthrough of screenshots taken under OS X 10.3.3 Panther and OS 9.2.2 to set them up for mutual file sharing using a crossover cable. (This procedure appears almost exactly the same for Jaguar(10.2) and Tiger(10.4) or OS 8.5 and OS 8.6 systems. The system preferences / control panels used are almost identical, and differences seem cosmetic.) If you prefer to see the screenshots for OS X 10.2 Jaguar instead, see this OS X 10.2 walkthru.
OS 9 / OS X.3 File Sharing
Using a PowerMac G4 as a server-ish machine?
Power Mac G4 as a Media/File/Print Server