Upgrading to 2000 domain.

Hi
According to MS these are some of the steps which should be performed when upgrading an NT4 domain to a Windows2000 domain:

The First DC that should be upgraded on an NT4 domain should be the PDC, the BDC(s) are to be unplugged from the network.
Before upgrading the PDC you should first disable WINS, DHCP etc, this is due to the database conversions taking place.
Choose to create either a Forest or a Tree (for three - five servers a Forest is recommended), decided on what the Damain name is and proceed through the upgrade, make sure you have a enough space where your SAM database is stored as it could expand a lot.
Decided whether or not to install in mixed mode or native, (mixed is so that if you have existing NT4 servers they can coexist with the Win2k servers) after finishing the domain upgrade you can then change the domain config to native whereby you get the advantages of the likes of Active Directory functions within the entire domain.

After the upgrade has gone toplan (with the PDC) and you are happy with it you can then attach it back to the network along with the BDC(s). (This is due to the fact that the SAM database is sought after by the BDC(s) at regular intervals).
Also because the BDC(s) do not havea writable version of the SAM it should be done on the PDC first, having the BDC(s) ofline while this is being done means that if all fails you can get your BDC(s) online and promote one to a PDC and you will still have a network.

That is what MSsay you should do, I do not know what will happen if you plug in a Win2k DC onto a domain that has a NT4 PDC already running, I would advise you to take down that PDC before plugging the Win2k DC then upgrade your other BDC(s) if you have any, the BDC(s) should be upgraded after the PDC due to the fact that they will be joining a forest or existing 2k domain.

Hope this is some sort of use to you.

Answer 1 is right on, but here's a little more info.
Win2K Domain Controllers CANNOT exist in a NT4 domain (which it will be if the NT4 PDC is still running). NT4 BDCs CAN exist in a Win2K Active Directory domain. Make sure you demote your NT PDCto BDC (offline) before plugging it back in to the network.