Migrating users accounts

I believe that the 2000 Server would need to be on a seperate Domain.

You don't need 2 domains for this.
Yes, you CAN do it that way. You could set up the Win2K box (separate from your NT4 PDC) with Active Directory, running its own separate Domain (again, different from your NT4 Domain). You could then set up a 2-way Trust between the 2 Domains.
Then you can run ADMT (Active Directory Migration Tool) and migrate the NT4 users to Win2K AD. Then bring down the NT4 PDC, reformat it, and install Win2K and make it a member server of the AD Domain.
You could do it that way. Nothing wrong with that.

But, you could upgrade your NT4 PDC to Win2K. That will, during the upgrade, install/configure AD, and it will upgrade the NT4 users to users in AD. All this would run on the current NT4 PDC, which would, when done, be Win2K AD (and be the Global Catalogue server, as well as PDC emulator, and every other FMSO role).
You could then build the 2nd server, install Win2K on it, add it as a member server to your one single existing AD domain. THEN you could run DCPROMO on this new machine, and make it a 2nd Domain Controller of your single AD domain. (NOTE: in Win2K and AD, all Domain Controllers are basically equals; no Primary or Backups now.)
THEN you could TRANSFER all FMSO roles FROM the old NT4 (now Win2K) box TO the new Win2K box. Then run DCPROMO on the old box, and remove it as a domain controller, leaving it a member server.
So, 2 options there.
Have fun!

Is the Mirgation tool an extra tool option I must install.
Because I have'nt found it yet.
thanks for your help.

ADMT 2.0 is indeed the tool if you are doing a parallel migration. If you are doing a in-place upgrade that gets more complicated as indicated in the above post.

1. Build the server as a NT4.0 BDC, pull the PDC off the network, promote BDC to PDC, conduct upgrade, rebuild old PDC.

I did a mixture of both Joseph and Chris' ideas and it went smooth. Instead of messing with my current NT4 PDC and trying to upgrade it, I installed NT4 on a standard PC as a BDC for my domain. I synchronized the domain and then promoted the personal computer to be the PDC. Once that was done, I resynched the domain again and then physically upgraded the PC to windows 2000 server exactly the way Joseph explains it. Now my PC was the DC for a windows 2000 Active Directory domain and had all my user accounts. I then took my new server that had a clean install of windows 2000 as a member server and ran DCPROMO on it and made it into a DC also. I then transferred all of the FSMO roles to the new server.I then formatted my old NT4 PDC and reinstalled it as a DC in my domain and then cleanly removed the PC from the domain. I know it is a bit of work but you get a domain controller that has a clean install and all of your user accounts at the same time.