The best way to take stock of your licenses is to call microsoft and have them look up and send you an excel spreadsheet with all of your licenses in it.
1-800-MICROSOFT
If you put in a 2003 exchange server and already have a 2003 domain controller, then you'd have to buy 2003 Exchange CALs. They are cheaper than the 2003 Server CALs.
Any and all microsoft licensing questions are better answered by calling 1-800-MICROSOFT.
From my experience:
CALs are backwards compatible. This means if you have 2003 CALs, they work for 2000 server. However they dont work the other way. Exchange needs its own set of CALs with the 2003 Server CALs you need for the server.