If you NEED to install OSX 10.5 on an unsupported G4 for some reason, know that there are a couple ways you can force the Leopard installer to boot on unsupported hardware.  Here is the Open Firmware way, allowing you to use a completely unmodified (and even a genuine) OSX 10.5 retail install DVD (this can also be done using an USB stick).  Before we begin, be warned that the reason Apple blocked pre-867mhz G4's to boot OSX 10.5 is...

To boot any* PowerPC Mac (e.g. an iMac G3, an iBook G4, a PowerMac G5, etc...) from an USB stick or drive, FIRST know that if your model is 300mhz or faster, then first refer to the boot from CD guide and if it does not work, then follow these instructions: * The Lombard PowerBook G3 cannot boot from USB 1. Connect your USB stick/drive in a root USB port on your PowerPC Mac. 2. Power it on (or restart it if it's already running) and...

If you need instructions on how to make a USB flash drive for this tutorial, one for creating it on Windows can be found here. Creating the USB drive on a Mac running OS X is as simple as restoring an image with Disk Utility. The Leopard disc image used in this tutorial can be found here. Note: This tutorial assumes you have already created a bootable USB drive containing Leopard. 1. Shut down your PowerPC Mac. 2. Plug in your Leopard...