So, I set out with an 8MB cache 200GB drive and a pocket screwdriver to see if it could replace the existing 120GB Apple (IBM) drive that came with the machine.
In the end, the drive was installed without significant incident.
The first chore was to figure out which drive was the one that I wanted to replace. I knew that it was /dev/disk0s9, but it wasn't clear which drive bay it was. After some investigation and some rapid disk I/O (intended to make the drive's light blink), I confirmed that disk0 was is in the rightmost bay.
I then unmounted the drive with 'umount' to make the system forget the disk that was in there. Then, I popped the disk handle and waited for the lights to all go out and removed the carrier from the front panel.
Removing the existing disk and replacing it was straightforward. Four screws on the bottom of the unit must be removed and then the plastic cover must be removed from the rear (connector end) of the carrier.
With the cover off, the drive can then be rotated upwards (being careful not to stress the power or IDE cables connecting the drive to the carrier) and then the cables can be unplugged from the drive.
With the drive now free, put it safely away and connect the power and IDE cables to the new drive. The orientation will be obvious, since the carrier is so tight-fitting.
Before rotating the drive into place, it is important to make sure that the drive's jumpers are set to Cable Select. This is the setting that my original drive had.
Now, rotate the drive into the carrier and fasten it down with the four screws removed previously.
Once the drive is seated correctly, secure the plastic cover.
Take the drive and insert it into the XServe and lock it in. At this point, the drive light will come on yellow and then settle to green (unless there is a problem with the disk.
Now, you should be able to format the drive using Disk Utilities and your drive will be accessible.