I was just checking out Apple's new iBook G4's online. Seems like another good possibility, since they have G4 chips running at 800MHz, 933MHz, and 1GHz respectively. No option for a hard drive spinning at 5400 RPM until you reach the 15" Powerbook though. If I had my druthers, I'd get the Powerbook.
In my continued use of the iBook 600MHz, I did run into a problem recording audio to the internal drive, but not all the time, and I don't think it was due to the Quatrro. If anything I suspect the slow HD. I've had no problems recording to an external firewire drive (120 GB, 7200RPM), which is a great option if you can swing it. It does however add to the clutter that you have to tote around if you do location recording.