My desktop computer spent all afternoon copying stuff from my old hard drive to my new hard drive, using Mike Bombich's Carbon Copy Cloner. Great tool for moving Mac OS X volumes' contents without losing track of which users own what. I just restarted with the new drive selected as the startup volume, and it came up fine. Dismounted the old drive to make sure it was finding everything properly.
I've now got 50 GB of new elbow room on my main volume, as well as a second 70 GB volume that's empty and ready to have stuff live in it. It's even a faster drive, which should make the computer a little faster in general. It's a dual 800 MHz G4, which is slow these days, but I rarely need it to do anything that overtaxes it.