If you are just adding a member server it’s no big deal. If you want to add a 2003 DC and have 2003 functionality then you would have to take some additional steps, but not for just a member server. Is this going to be a DC?
No procedures are necesary at this point you can add the Windows 2003 server to the domain just as you would a regular PC.
When you feel you want to promote the server to a DC, this is what you do:
1. Update all of your Windows 2000 Domain Controllers to at least SP2.
2. Prep the forest and domain
a. Open command prompt and go to the i386 folder in the Server 2003 CD. Type: ADPREP /forestprep
b. Once the forest is prepped, type: ADPREP /domainprep
3. Promote the server to a DC. Run DCPROMO.