I avoid Leopard on any machine that doesn't have Core Image support, with one glaring exception: the 1.0ghz TiBook. The L3 cache, at least in my experience, makes up for that somewhat, and it's actually a pleasure to use. I *do* get some glitches; Appcleaner's 3D mode does some weird things when I use it; but not enough for me to worry too much.
CI definitely makes Leo BETTER, but at the same time in some ways a high performance non-CI card can do about as well as a low performance CI card.
The classic example I always think of is the GeForce FX5200 vs. the GeForce 4Ti-which one I'd rather have in Leo depends on what exactly I'm doing. The Intel GMA950, which wasn't even used in any PPC Macs, is also CI capable but probably even more of a step down than the 5200.