With Windows 2003 R2 you can set up the DC as a remote office DC, and thus saving on bandwith since it will sync the DC information on a schedule and not continuously.
The AD can be on the same server as a file server (since AD is actually a file server role), but the important thing is that the AD does not have a web server on it.
AD is disk and network intensive (not in high bandwith, but short quick burst) like a web server, and the web server add security concerns. Another service you don't want on a DC is email server.
that Aside, you can setup the W2k3 server as AD/File and print server with a schedule to sync. This server will also have a GC, so the end result is that your remote clients connect to the remote DC and the remote DC syncs ourside peak hours.
MS has information about the remote/branch DC deployment on their web site (I think in the technet section)