I just swapped out the meager (wow) 20 Gb hard drive that came with my G4 Cube with a $160 7200 rpm 80 Gb drive (from APS Technologies). The swap out was easy enough, but the prep was tough. A friend warned me that you can't use Retrospect for X to restore to an empty hard drive. In fact, you not only have to back up all your files using Retrospect (to capture all the tweaky OS X stuff), but you also have to install 10.1.x on the new drive up to the release version of your backed-up volume.
So I did. I pulled a FireWire 60 Gb drive in from home, make Retrospect backups of my two partitions, rebooted to the FireWire drive, initialized and partitioned the new drive, and installed 10.1 on it. Then ran through the updaters to get up to 10.1.3, and then booted from that new drive to run Retrospect and copy everything else.
Miracle of miracles, after rebooting following the copy, my machine is pretty exactly as I left it. With a 30% faster rotational drive with four times the storage. Thanks, Apple!