I usually boot from CD but from my notes you should be able to boot from USB using OpenFirmware (cmnd-opt-o-f) with something like this:
Sometimes it is usb1 other times it is usb0.
Other than that, if you are going for PPC, I would recommend to use Lubuntu instead. And since you are wanting to install a server, I would recomend using the Alternate Install CD and choosing to choose "Install a command-line system" installation. It's there in the initial text menu available under the F4 key.