how much time do you have pollard? you may want to take a step back if you can afford to and come up with an upgrade plan. you'll very easily get lost going back and forth between setups if you haven't done this before. also, since you haven't added the new server to the domain yet, feel free to blow away the installation anytime if you don't feel like you have something done correctly. the repetition will help you later on. i've built a couple of networks from scratch, and we're currently looking at upgrading our current 2003 infrastructure to the 2008 arch.
i agree with rebate, you should create a new domain with the 2008 box and a couple of test clients. then either create new accounts on the new domain, migrate your email over to the new exchange server and get all of your server apps installed to make sure there's no compatibility issues. then add a "test" users (actual users) to the domain and have them test run your new network setup for you. once you feel comfortable with the new setup, begin migrating your users and their desktops to your new domain and retire the old one.
this way you don't lose functionality of the 2008 server you have an entirely new domain and you have the notes from the install and build out at your disposal.