Good post, but in my experience the situation is more like:James Philp said:Speed:
9<10.2<10.3<10.4
Memory:
10.2<10.4<10.3<9
Speed on G4: 10.2<9<10.3<10.4
Speed on G3: 10.2=10.3<9 (at least on slower G3s, haven't tried Tiger yet)
Memory: 10.4<10.3<10.2<9
9 of course runs ok with under 100MB; 10.2 needs around 256MB, 10.3 feels better with at least 384MB, and if what people are saying is true Tiger will probably run the best with at least 512MB.
It's not even worth thinking about 10.0 and 10.1--they both probably need a little less RAM than 10.2, but are dog slow on any computer.