I installed Leopard on two older machines that are not officially supported by Apple.
PowerMac G4 533Mhz 354MB RAM: This machine works okay with 10.5. It is somewhat slow with graphics and the low memory causes it to hit the disk a lot. The most serious issue is waking from sleep, which results in a kernel panic. I tried adding more RAM, removing SCSI card, disconnecting an older drive, but the kernel panics continued. I’ve set energy saver to never put the machine to sleep for now.
iMac G4 1Ghz 256MB RAM: This machine only has half of the required 512MB RAM for 10.5. It seems to run fine, but swaps to disk a lot.
When spotlight first indexes the drive on first boot, the machines crawl because the disk is so busy with swapping and indexing, but once that is finished, the machines are usable.