I got my computer back today. I don't know if it is fixed; I'll have to wait and see if it freezes over the next few days. Apple didn't say what was wrong; predictably, they just noted that they replaced the logic board (and yes, I still have 512MB on it
They reinstalled Mac OS 9.2.2, apparently over the existing installation (I see the telltale Desktop icons for accessing email and the web, and the Late Breaking News alias in the volume). I don't know what possessed them to do this, but they didn't say what software changes they made to my system, so I have to erase the whole thing and reinstall from an image I made before shipping it off. That's really stupid; if they are going to change my system, they should at the very least tell me exactly what they changed. Sigh. I have no idea whether they added something important (doubtful, since it was a hardware problem, which also makes me question why they bothered with this in the first place), and since I heavily customize my system, they likely added back or removed things I didn't want added or removed.
Well, I'll be happy if the computer isn't freezing anymore. To be continued (hopefully, for not much longer)