I have a Dually 1 GHz Quicksilver running Leopard. It has 1.5 G of RAM. I also have 3 HD's installed, one is a 1TB SETA (I got an adaptor that allows the SETA to connect to an IDE ribbon). Total GB's is 200 GB, 300 GB and the 1TB, total 1.5 TB.
I have access to all the HD's, not limited in space.
While a bit slow, it does everything I need it to do. Assuming your G4's can use larger HD's, they make great machines.