Run dcpromo on the new machine after installing Windows 2003 Server.
It will ask you if the controller will be part of a new domain or an existing domain. Select an existing domain and give it your domain name. Finish running through the wizard. After you finish making it a DC and reboot, install DNS on the server to make it a backup DNS server for your domain.
following info from microsoft technet site is quite useful on this:
Creating additional domain controllers
If you already have one domain controller in a domain, you can add additional domain controllers to the domain to improve the availability and reliability of network services. Adding additional domain controllers can help provide fault tolerance, balance the load of existing domain controllers, and provide additional infrastructure support to sites.
More than one domain controller in a domain makes it possible for the domain to continue to function if a domain controller fails or must be disconnected. Multiple domain controllers can also improve performance by making it easier for clients to connect to a domain controller when logging on to the network. You can add additional domain controllers over the network or from backup media.
Before adding domain controllers you should thoroughly understand Active Directory and the requirements necessary to set up additional domain controllers in an existing domain.
With Windows 2000, the only way you can create an additional domain controller in an existing domain is by replicating the entire directory database to the new domain controller. With low network bandwidth or a large directory database, this replication can take hours or days to complete. With servers running Windows Server 2003, you can create an additional domain controller using a restored backup taken from a domain controller running Windows Server 2003. This backup can be stored on any backup media (tape, CD, or DVD) or a shared resource.
Using restored backup files to create an additional domain controller will greatly reduce the network bandwidth used when installing Active Directory over a shared resource; however, network connectivity is still necessary so that all new objects and recent changes to existing objects are replicated to the new domain controller.
It is recommended that you use the most recent backup available. Older backups require more network bandwidth for replication. The backup used cannot be older than the tombstone lifetime of the domain, which is set to a default value of 60 days (180 days in a forest that is created on a server running Windows Server 2003 with Service Pack 1 [SP1]).
If a domain controller that was backed up contained an application directory partition, it will not be restored on the new domain controller. To manually create an application directory partition on a new domain controller, see Create or delete an application directory partition
When adding an additional domain controller using backup media, a System State backup taken only from a domain controllers running Windows Server 2003 can be used once it has been restored.
Step-by-Step Guide to Setting Up Additional Domain Controllers.