Following copied from experts exchange
You cannot run an NT 4 domain with a Windows 2000 domain controller. If you load W2K server then run DCPROMO, the wizard will install Active Directory. There is no way to make it an NT 4 dc. Also, if you do not update the existing PDC to W2K and Active Directory, you will have 2 different domains (assuming you made the W2K server a domain controller with a domain of the similar name). This will cause all kinds of problems with users trying to access resources on the existing NT domain.
You are probably thinking that you can still have NT 4 domain controllers in the domain. This is true. You would be operating in mixed mode (OS levels of the domain controllers are different). You need to remember that the first W2K domain controller installed will take on the role of PDC master, which is the role assigned to maintain downward compatibility between OS levels. Your NT dc's will all now be bdc's and like before will have a "read-only" version of the domain's SAM database. (One of the powers of W2K and AD is that all domain controllers have a writable version of the SAM.)
There are tons of articles on the MS technet site about upgrading from NT to W2K or W2K3. Go to support.microsoft.com and start searching.
Andy Leates MCSE CCNA MCP+I