OK here we go.....
I assume you have a 2003 Server fully patched and joined to the domain.
1. You have to prepare the Active Directory if your going to migrate over to 2003 from 2000. This tool is called ADPREP. You must prepare the:
GPO
Domain
Forest
before adding in a DC. This will prepare the 2000 Schema for a new DC (2003).
http://www.petri.co.il/windows_2003_adprep.htm
Then you run DC promo on the new Server and add it to the existing domain.
2. If you intend to decommission the old DC you must transfer all the FSMO roles to the new Server.
http://support.microsoft.com/kb/324801
3. You will need to make the new server a Global Catalogue Server.
http://www.computerperformance.co.uk/w2k3/w2k3_global_catalog.htm
4. Service Transfer- Pause DHCP on old server, copy settings to new Server, start service on new, stop on old. Install DNS on new server. (ON new DHCP server specify both DNS servers whilst migrating)
5. DC promo old server and decommission.
6. Change DHCP to reflect new DNS server.