You'll need an ATA-133 PCI card to use the whole drive; some ATA-100 cards and Firewire cases also support large drives. Otherwise, your G4 IDE bus has a 128gb limit. Intech just released new HD Speedtools software, which will allow you to format and see all of it in OSX, but you're still limited to max. partition size of 128gb.
You might also want to consider a SATA PCI card and a SATA drive.