The standard ATA drive that comes with a G3/G4 is well suited for audio. Adding a 2nd ATA Drive is a good option if you want more space for projects.

Another option is to get a SCSI drive (and SCSI accelerator if you Mac isn't SCSI equipped). Ultrawide and UW2 are good SCSI protocols for audio.

For either ATA or SCSI drives, 7200 RPMS or better, with a 9ms seek time or better would be the spec to look for when purchasing an additional drive.

Motu has been able to obtain good results with Firewire drives as well. Glyph makes high-end firewire drives for audio, for example.

USB drives aren't recommended.