Re: Adding new 2003 Server
Basically, run adprep /forestprep; adprep /domainprep; run dcpromo.
There's a good article at http://www.petri.co.il/windows_2003_adprep.htm with step by step instructions - that web site has masses of other useful info - well worth checking out.
If you want to remove the old server then make sure you transfer the FSMO roles to the new server first and (most important!) make sure you make the new server a global catalogue server otherwise it won't service logons.
I really would try and keep two domain controllers on your network, though. It's very hard work to recover a domain if the single DC dies. If 1 DC in a multi-DC domain fails then you just wipe, reinstall and dcpromo - nothing is lost.