You absolutely need a G4 to run Leopard from what I understand. Luckily you can drop in a G4 ZIF as long as it's 66 MHz bus compatible.

The oldest I've heard is Leopard on a G4 upgraded B&W. It's easier because it's New World. You would need to somehow use XPostFacto, or do whatever XPF does, as well as what the guy did for his B&W, and maybe it would work. If you got it to work though you would be the first.