You could always booth the G5 into "firewire target disk mode" (if it's supported). This is how I installed OSX 10.5 onto an older macbook that had 10.4.
I put my 10.5 DVD into my Mac Pro, connected my macbook in firewire target mode, and it installed OSX onto the macbook pro. This will take your main computer out of commission as it uses the main computer to run the installer, but copies the files to the "target" computer.
Worked for me.