I think if you can run Tiger you'll be fine with Leopard.
Hell my PowerMac G4s (from 2000 and 2001) still work fine in Tiger and I have no doubt they will run Leopard just as well... its not like Apple's OS updates are major changes, at the core, its still the same system.
However if you have less than 512MB that might be a problem, all my Power Macs have more than 1GB or 1GB.
Should be fine - you might not get all the flashy eye-candy but it'll work. I fully expect it to run just fine on my 1.2GHz iBook - just want to max out the RAM before I install.