Basically, if you have a G4 Mac, you should be able to install Leopard (OS X 10.5). Apple has chosen to set the minimum system requirement to 867 mhz G4, which excludes a vast number of G4 Macs. With a little patience, Leopard can be installed on these old machines.
- Download Leopard Assistant and run it from your old operating system (presumably Tiger, but also works in Panther).
- Once it boots in Leopard Installer, change the option so that instead of a "Upgrade", you select "Archive and Install".
Unfortunately, Snow Leopard is Intel-only, so Leopard is as far as the old PowerPC G4 architecture can be upgraded to.