Um, it probably would have worked if you had just double-checked the jumper settings first. This is of course a pain in the ass because it's a Dell, but that's because Dell makes awful cases. If you were working in a standard ATX case, things would be much easier (though not nearly as nice has having a PowerMac G4 with the drop-down door).
FWIW, I've had my G4 not recognize my hard drives before, and it was due to the jumpers being set improperly based on the drives' locations on the cable. The middle connector goes to the slave drive, and the one at the opposite end from where the IDE cable connects to the motherboard is for the master drive. Then you have to jumper them properly. Once that's done, you're good. If you haven't checked your jumpers yet, you should.
If you have... well then, allow me to begin working on extracting my foot from my mouth...