There are a bunch of gotcha's but in general this should work:
First, run a program, found on the Windows 2000 cd, don't exactly remember where, called DSCLIENT.EXE on the Windows NT4 server. This gives the NT 4 server the ability to connect to Active Directory.
Next, make sure you are patched up to NT 4 Service Pack 6a, which contains all kinds of other 2000 fixes and patches. I usually finish it off by connecting via the Internet to Windows Update (Ie explorer, Tools, Windows Update) and apply any final patches or critical updates. I also recommend making sure you are running the latest version of Internet explorer on the machine as well, because I ran into a problem once where NT needed a IE component and the NT server had some antique version of IE and this somehow affected the operating system.
Next go to Network properties on the NT4 server, go to Network Identification and change the domain name, and enter any changed TCP/ip addressing if necessary.
Next connect it to the new domain and go to the 2000 server and create a computer account for the NT server. If 2000 can't find it, you've got a network setup/TCPIP problem.
If 2000 finds the NT server, go back to NT 4 and try to log in.
On the client side, after that you will have to remap the M: drive on all the clients if your on PSQL. If you have any problems, reinstall the client.
If you are on MSsql, I would guess that you would have to re-run the client install programs from the nt4 server as well.
If things don't happen for you, let me know and I will try to help you.