I wouldn't use any less than 10.4 for anything. Tiger was the longest lived single version of OS X (it spanned the PPC/Intel transition, for instance) and there are at least a few diehards out there still "supporting" it. As to performance, about the only element of it that made it any "heavier" than 10.3 was Dashboard; it's easy enough to turn that off if you really don't want it sucking up some RAM. Other than that I recall it being mostly superior to 10.3 performancewise.
(Well, okay, "Spotlight" also can be a pig sometimes, but if necessary it can be tweaked as well.)
So far as I'm concerned Leopard is a cow on anything with a G4 in it. Granted I only base that on a single sample install, but the machine in question was a 1.5Ghz Powerbook, they don't come much faster than that.