I also have a g4 in my powerbook. As to performance, many people claim that tiger runs better on their machines than panther did. Apple seems to streamline things with more experience on each platform. As to leopard, given the intel/ppc switch, we may see less of that, but no one really knows. I'd just wait till it comes out, see what others' experience is on older machines, and make a decision then.
Cheers