Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
A IDE hard drive should work. Although i don't know how large of a drive it can handle. I don't know if that is hardware or software.

Newegg.com or zipzoomfly.com
 
Asuma - you are limited to a 120GB hard drive on the older G4's.

The later Quicksilvers and then the Mirror Drive models can support the higher capacity IDE drives.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.