Dewboy:
This document is how to set up an additional domain controller. As I understand your requirements, you need a separate domain for testing. Thus you would essentially start fresh. You can use the same subnet for the new domain, but for several reasons I would suggest a different subnet.
As I mentioned earlier, a child domain would create a new sub-domain for the isolation you need, but allow for share forest resources. If you are setting up a test exchange environment, I believe you would need a new forest.