Upgrading NT domain to Active Directory

We will be upgrading our network from an NT domain to an active directory domain. We are fortunate enough to have a new server to replace our PDC. It was recommeded to us to install NT on this new server and upgrade it, we will be installing Windows Server 2003. Most machines are Windows 2k and Xp pro, we have a few NT and 95 machines, about 120 total. What issues can arise especially with authentication, login scripts, policies, or any unusual stuff? I realize this is a pretty broad question, but all input is appreciated.