You can do one of two things:
1) Get a Mac OS X 10.5 install DVD, boot it up from there and do a fresh install of Leopard. Then connect a USB hard drive to it and go to System Preferences and turn on sharing, then share the drive out over your network that way. (Turn on AFP or Windows Sharing (Samba))
2) Download a copy of Ubuntu for PPC and install Ubuntu on it. Connect a USB hard drive and share that out.
Option 1 is more or less what I do at home with my Intel Mac mini. I have 5.5TBs of storage attached to it. It is plugged into my router by ethernet. All of the machines in the house can access any of the drives because of the way I have shared them out. I have both AFP and Samba enabled.
Hope this helps
pac