For this machine you can find a 120gb 5500rpm drive or a 100gb 7200rpm drive faster one is by Hitachi, 5500rpm drives by Seagate and some others.
IDE drives are hard to find anymore and I don't think you'll find a larger internal drive that will work in a G4 Powerbook.
You can use a larger SATA drive as an external second drive in a FireWire external enclosure by MacAlley and have a boot able backup, too.