I have Leopard on my 800 MHz iMac (with nVidia graphics) just fine. Although nowadays, there really isn't much reason to run Leopard over Tiger. Most modern software has abandoned PPC altogether, and most "vintage-specialized" software (like TenFourFox) specifically targets Tiger. I'd just stick with Tiger. (I haven't booted Leopard on the iMac in ages, I just use Tiger now.)

Target Disk Mode was my install method, (using an 867 MHz PowerBook G4 as the active system, no less.) I used software update to go to the latest 10.5.8 no problem. (Although I did that while still on the faster system through Target Disk Mode.)

Also, I *HAVE* used Leopard on a 400 MHz G4 - an upgraded-to-dual 1 GHz G4 Digital Audio whose CPU card failed, and at the time a 400 MHz was the only spare I had. It worked, but was dog slow. (It did have 1.5 GB RAM, though.)