A friend sent me a message today to say that this site was down with some cryptic error message. In the course of solving that mystery, I found that I could delete something like 200MB of data from the database. Doing so took several minutes, but once I did I was able to successfully upgrade the old Drupal from 4.x to 5.x and then from 5.x to 6.12. I've spent a couple hours lately trying to do the upgrade and am happy that I've finally accomplished it.
Boring details follow.