Well I'm nearing that crossroad Alan...Spend $325 for a 1.8 Ghz CPU upgrade, 1 GB bump of RAM, and a Pioneer Superdrive or sell and spend $600-$700 for a used G5. My goal is to be able run OS X 10.4 or 10.5 (so that my daughter's silly iPod nano video will work) and run iTunes/Internet/Videos/Pictures. Nothing fancy but I've grown sorta attached to the G4 box...
Edit - so far I have $50 in the G4. The SATA PCI card works great. I took out the dinasour drives and the machine runs really quiet. I also added a USB 2.0 PCI card for all the devices and it works great to.