As long as you keep video-memory-eating features like high-detail textures, trilinear mapping (which has to be emulated on the iBooks graphic card)etc. off, you will be fine. I would still advice to run it in 800x600, then it will run like a charm!
I used to play Q3 on a 400 Mhz iBook and in 800x600, with med-res textures and some other tweaks, it was great.