i have an iBook G3 700mhz, and a dual 1.8ghz G5.
considering one is about 4-6x as fast as the other, the OS is virtually indistinguishable. this isn't to say that the G5 is slow, but that the OS is so well optimised that it runs *amazingly* on my old G3 (Tiger). that's the impressive part, that a 6 year old computer can run the lastest and greatest [well, until october, anyway] smoothly and snappily.
the real difference is in video and image handling. a quicktime video export that would take ~20 mins on the G5 takes roughly 2-3 hours on the G3. and that same video encode would probably take about 3-4 mins on a new Core 2 Duo.
the OS shows very little difference.
Dual 1.8GHz G5 2GB, 1TB, Radeon 9600XT 128MB, 10.5
20" Apple Cinema Display + Dell 2005FPW 20" dual-head
iBook G3 700MHz 640MB, 40GB, Rage128 16MB, 10.4, dying battery