Another way to go (the one I would probably take, being partial to having a new domain with a pure Windows 2000 machine and not an upgrade) is to upgrade the existing PDC to Windows 2000. Then take the Dell machine and promote it to a Windows 2000 DC. You will then have to move all of the FSMOs (Flexible Single Master Operations) from the old PDC to this new server. Then you can get rid of the old PDC and all of your users and accounts will be there.
Risks are greater with the second option. First, if you want to fall back to NT 4.0, you had better have a good backup tape because you won't have an unaffected BDC like you had in the first option. Also, moving the FSMO objects can be a little strange for a newcomer. But following some directions from the KB article, Using Ntdsutil.exe to seize or transfer FSMO roles to a domain controller (Q255504), should clear up any huge stumbling blocks. You will be transferring the roles. Oh, and don't worry about the issues of being a Global Catalog server -- since this is your only DC, the problems don't apply.
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