Again, maybe old news but I thought I would post something interesting that happened during a recent upgrade from SPS 2003 to MOSS. SharePoint is very finicky when it comes to database transactions and sizing during the gradual upgrade process. Here are a couple of things to note:
1. Make sure you have enough size - When upgrading a site that had about 150GB in content, we needed about 500GB in space to complete the migration.
2. Make sure you know where your _Pair DB's Are - SharePoint for some reason didn't put the _Pair db's on the default data and log file locations (C:\) and had to be moved to a larger volume. This caused the upgrade to fail with space issues.
3. Make sure to set the _Pair data and log files to AutoGrow - Now that you have the right amount of space, make sure that you configure the new databases to grow correctly. Large content databases will have large transactions when copying the sitecollections. This can also lead to things like timeouts and space issues. You should also consider setting the recovery model to "Simple" during the migration to keep things tight.
Just some things to think about.