I think you'd be OK going over to Tiger. I've heard it said that each new release of OS X (given enough RAM) has yielded better performance. I can't say for sure, however, since my MBP is my first Mac since an old IIsi, running OS 8.
I have a 350-gigahertz G3 tower that a relative used for three years or so. He received it with OS 9 and Panther, and trashed 9.
It's no world-beater running X, but it runs surprisingly well. Then he upgraded it to Tiger, and it ran even faster. The speed increase was easy to see. Tiger probably is better optimized than Panther.
I got the G3 back not long ago when he bought a MacBook. I'm thinking of putting 9 back in it and networking it to my G4 so I can pull 9 off this one.
Leopard might run faster yet on the old G3, if support isn't dropped. It would be interesting to find out.