For any DAW application, you're going to want the processor power and extra ram. And I imagine you're going to do multitrack recording (not just midi sampling) with Digital Performer.
In all honesty, I've never used DP4 on a notebook, just the desktops. But I imagine it would run well enough (as long as you don't plan on doing realtime software effects with your mixing) for 15-20 tracks. The biggest thing hampering it with the ibook is the 256 of ram.
