1) install windows 2003 and add this server as a member server of the existing domain.  (simply join the domain)
2) the DNS of this new server (in TCP/IP settings) should be pointed over to the existing DC/DNS server.
3) run DCPROMO on the new windows 2003 server and choose to join an existing domain.  Allow the wizard to complete.
4) Wait an 20 minutes for synchronization to complete... or you can force sync.
5) install the DNS service on this new windows 2003 server.  (in control panel --> add/remove Programs --> Windows Components --> DNS)
6) add the IP address of the new Windows 2003 server to all servers and clients TCP/IP settings.
7) Make this new windows 2003 server a Global Catalog server: http://technet2.microsoft.com/WindowsServer/en/library/7b1c3e1c-ef32-4b8e-b4c4-e73910575f611033.mspx?mfr=true