Or if you own/can borrow a PPC sytem that DOES support Leopard, you can; boot up the G4 you want to install it on in FireWire target disk mode, connect it via FireWire to the Leopard supporting PPC Mac, boot that computer up from the Leopard install DVD, and select the hard drive in the non-supported Mac as the destination. Thats how I got 10.5 running on my Cube.