DANGER WILL ROBINSON!!!
The Tibook 550 and 667 models are **especially** unsuited for audio use - in any app. This is due to the lack of Memory Bandwidth in these 2 models. A older Tibook 500 or a newer generation 667/800/1gig Tibook (the ones with a DVI video output connector on the back) are your better bet.
Basically Apple got it wrong with a new CPU in the 2nd generation of Tibooks. In the real world this means you would get far less tracks and FX than you would on a 1st, 3rd or 4th generation model.
That said, I currently use a Tibook 500 which I'm pretty happy with. The main thing it has over the iBook (apart from speed) is a PCMCIA card slot -
which is your best option for a high quality audio interface such as the RME Hammerfall or VX Pocket. (Both relatively expensive, but good. Using the cardbus taxes your computer much less than firewire or USB)
hope this helps