I have a Windows 2000 server and I want to add a Windows 2008 Server server to the forest.
Do I add the Windows 2008 server to the domain before running
DCPromo or do I leave it off the domain?
It does not matter. Since it saves time to DCPromo directly, do that.
MINOR CAVEAT: The SID of the Server 2008 install should be different than any of the existing machines. If you didn't clone this from another install (as in a virtualized environment), then you should be fine.
I'll put this in as an answer since it is fairly important.
You'll need to prepare the forest using adprep before adding the new DC.