Well, I've had Leopard on a 12" PowerBook, and its FX5200 isn't very... good at all. I found Leopard's GUI to be acceptably zippy, but noticeably "heavier" than on my 15" or 17" PowerBooks with a fully CI-capable Mobilty Radeon 9700. So I'd be inclined to say the Ti4200 should be better than the bottom-of-the-barrel FX5200, but I'd look into turning off all the graphical niceties such as the 3D Dock (the translucent menu bar is turned off automatically on non-CI GPUs).