Make a backup copy of your AD before you go any further.
Install your new server, and join it to the domain as a member server. Before you can run dcpromo on the new 2008 server, you must run adprep on your schema master, to prepare the Active Directory schema to support Windows 2008 domain controllers. The installation DVD contains a directory called sourcesadprep. Go there and run:
After that you may run dcpromo to promote the server as a domain controller.