I use G3 350 B/W tower 640ram MacOS 9.2.2 with Cubase 5.1 or Logic Platinum 4.8.1 without problems. ( I think the same for Logic 5.).
Should be the same for you if you gonna use sequencing with a few audio tracks, fx,s and vst.
Cakewalk has not made to the Mac platform yet.

The only difference with the faster ibook or even faster computers is that you can run more audio tracks, more quality fx,s and more power vst plugins.
OS X also runs much better if you gonna use Cubase SX and Logic 5.3.
But the plugin front on the OS X is still under deveopment.

Which ever way you decide, You still need an extra firewire hard disk for better audio performance.
( Mind you Im still looking to buy one myself.).