G5 drives were SATA, the iMac G3's need PATA drives. I have a 40 gig with Tiger on it let me know if you want.
Also I think the optical drive fix for the G4 Cube works on the iMac G3, it's an adapter of some sort that makes it possible to use a modern laptop hard drive with the iMac. Also is this a slot loader or tray loader?