What is ATA, SATA and IDE?

SATA is Serial ATA which is the standard used by the built-in interface of the PowerMac G5 and the iMac G5.

ATA/IDE is the old hard drive standard used by G4s for internal hard drives. It is a Parallel technology.

According to Computer User's Dictionary:

http://www.computeruser.com/resources/dictionary/noframes/index.html

ATA is:

Advanced Technology Attachment. The specification for IDE interface.

IDE is:

Integrated Drive Electronics. Interface for connecting additional hard drives to a computer

You can put the ATA/IDE drive you have inside a Firewire drive case which accepts ATA drives such as the ones from Granite Digital/Relax Technologies, or get a PCI-ATA card to connect your ATA to your G5. There are companies which make special mounts to put ATA drives close enough to the PCI-ATA card so that the cable will fit in the case. I couldn't find any with a quick Google search, though you may be able to find some looking at http://www.xlr8yourmac.com/ Also companies like Sonnet make a Parallel ATA to Serial ATA bridge:

http://www.sonnettech.com/product/tempo_bridge.html

Yeah, that's what I do with previous drives.

I've got a silver bullet firewire case that I can just stick various things into when I see fit - handy item to have around...