If you have a Mac with less than 867 Mhz CPU speed and you want to install leopard, now with LeopardAssist you can do it. It's very simple. You only need to execute de action and put the Leopard cd on the Mac an reset the computer. If apple don't recomend install it on this computer is for something. I've installed it on a PowerMac G4 @ 400 Mhz and it runs really slow, I think Tiger is a great O.S. and you can use it with this macs.
Another way to do it is restoring a Leopard image on the unsoported mac and boot the image restored.
Leopard Assist Web Page