Migrateing from NT

We are about to start our migration from Windows NT to 2000. We currently have a PDC and 2 BDCs. I would like to migrate one of the BDCs to 2000 as part of the first phase of the migration. What are the problems and issues I need to keep in mind? Should I upgrade the PDC first? Should I upgrade both the PDC and both BDCs at the same time? What is the best way to upgrade all the domain controllers? I am just looking for some insight from someone who has completed this type of a migration.