you can just upgrade the current directory structure to windows 2000 active directory. i would suggest that first you make sure you have a full backup of each server before you begin. To make it a little easier bring up a brand new BDC for the current NT 4 domain, and once it's up and all domain information has been replicated, then upgrade it to windows 2000. During the upgrade it will ask you about installing AD. The active directory wizard takes care of installing a DNS server for you that will handle dynamic updates and SRV records. If you already have a DNS server in place, you'll need to take a few extra steps to get it ready for active directory.