A friend of mine ,who also has the powerbook G4 12 inch, asked me if there were any faster hard drives for his machine, as he was having trouble with logic audio performance. After googling a little I found a nice choice:
HD AT 2.5inch 60GB Hitachi Travelstar 7K60
IDE, ATA5, 7200 RPM, 8MB
The price for this piece of hardware is around 150