Although the hard drive bay is served only by a SATA bus, there is an ATA bus going to the optical drive. I installed the hard drive in the one available hard drive bay. I removed the optical drive so I could get to its cables and jumpers. To provide power for the hard drive, I installed a Y splitter on the optical drive's Molex power cable.
The short IDE cable that runs from the motherboard to the optical drive was replaced with a 36 inch ribbon cable. The long length was necessary to reach from the optical drive to the hard drive. Because I couldn't get a custom IDE cable made locally, I had to use an off-the-shelf cable. These cables assume that there is a long distance from the motherboard to the first drive and a short distance between the drives. This is the opposite of the G5 situation, so I had to install the cable backwards; that is, the end that normally goes to the motherboard was plugged into the hard drive. This means that the cable select configuration option won't work, so I set the jumpers on the optical drive and hard drive to Master and Slave.
The 36-inch length allowed me to fold the ribbon cable and tape it to flat surfaces so it will not block any airflow. I've had no problems with this installation. I'm curious what others think about this setup.
[robg adds: Proceed at your own risk ... my G5's only getting standard components, at least until its warranty is up. After that, though, I may re-visit this hint for some more cheap storage space...]

