First thing
backup the PDC that you are gonna upgrade to win2kserver, windows has a good back up utility for this.
i dont know if you have any BDC's but i would recommend setting one up if you dont, just in case you have problems with the upgrade.
that way if the upgrade has a problem, you can promote the BDC to PDC and save yourself some headaches
I would assume that you are running nt4 for your PDC, and nt4 can be directly upgraded to win2k server as part of the installation options
justmake sure you back up all system state data (user accounts etc..) and get a BDC ready for promotion should you need it
nt4 should have a utility for backing up data, and then use the system restore wizard once you have upgraded to win2k server to get all your system state data on the new server