If you have a "simple" nt4 domain containing PDC and some BDC´s there shouldn´t be such a big hazzle to upgrade. Take offline one BDC in case of catastrofy and then upgrade existing PDC to w2k. It should automatically became DC for your domain. Correct any issues and go ahead upgrade the rest of the servers to w2k. Add them as member servers in the domain whith the exception of at least one server that should became additional DC for redundance.
This is the "smple" answer and if your domain is complex you really should do some research before upgrading.
Edit w2k=windows 2000 server