Mac OS X v10.5 Leopard requires:
A Mac computer with an Intel, PowerPC G5, or Power PC G4 (867 MHz or faster) processor
512 MB memory or more
A DVD drive for installation
9 GB of available disk space or more
As long as it's not the earliest G4 12" or the 15" before the 12" was released, it should be fast enough. Maximize your RAM if you haven't done it already, as that will make a huge speed improvement for 10.5.