My own experience
I have a circa-2000 G4 desktop and when my internal SCSI drives blew, I decided to try IDE drives since they seemed to be a better deal per megabyte.
At any rate, the G4 has an internal IDE bus so hooking up the drive shouldn't be an issue. One caveat, though, is that the IDE address bus on mine (and presumably yours) can only address 128 GB of disk space.
I got around this, however, by downloading and installing [link|http://www.speedtools.com/ATA6.shtml| this driver]. This lets you use drives over 128 GB. (No single partition can be greater than 128 GB, though.)
Read over the FAQ before you get it, though, to make sure this will work for you. I'm currently using a 300 GB IDE drive with this and haven't had a problem.
Tom Sinclair
"You know what the chain of command is? It's the chain I go get and beat you with 'til you understand who's in ruttin' command here."