well that depends now if you want to continue using OS X or Win or switch to Linux.
The reason why I never went to Linux systems is that while Blender runs quicker there
the armada of software I use on the OS X side are not available on the Linux side.
If you are used to a G4 chip you would love any entry level laptop from apple to be honest.
Even the out dated Core 2 Duo would outperform your G4.
U could also get a i5/i7 laptop entry level and run VirtualBox in windows and virtualize OS X in it.
Alltaken got a Sony F series Laptop and he is very happy with it.
I think there is not much you can do to get the most out of Leopard. The system itself is when freshly
installed fast. I would try not to have any unneeded background tasks running. But those would only
generate fractions of speed-ups to be honest.