The goal is to get the most out of this 4 1/2 year old G4...without screwing it up.
Well, a brody and I can both assure you that if you install Leopard on your existing drive, you'll be screwing it up — soon, if not immediately — by overfilling the hard drive. If you aren't going to replace the drive, delete enough files from it before installing Leopard so there will be a minimum of 15GB of free space on it afterward, and maintain that minimum amount of free space at all times. If you don't keep at least that much free space, Leopard will run slowly, and if you allow the free space to diminish to significantly less than that, it will slow down further and become unstable, jeopardizing the integrity of your data.
As a brody says, there is no assurance that Leopard will speed anything up on your computer. But you can be sure it will demand more of your RAM at all times, and that it will produce the opposite of the desired performance improvement if you don't give it enough room to breathe.