This is not the place to get enough information to do what you're trying. The question you are asking shows a lack of understanding of Active Directory fundamentals. You're undertaking a major network overhaul and you apparently don't realize the implications of your actions. I'm not trying to insult you, but you're scaring us.
As soon as you add the first AD server to the domain, you're committed, ready or not. You're better off not upgrading your existing domain. In the long run live will be much better if you build a new one from scratch. At a minimum you need to do additional planning, and it doesn't sound like you have developed the skills to do it. If you can't afford a migration consultant, attend Server 2003, AD, and migration training classes. If you can't afford that, get one general 2003 book, one book specifically on AD, and one more on migrating. None of them should have the words "For Dummies" or "Idiot's Guide" in the titles. If you can't afford that, start trolling Microsoft's web site. They've got white papers on NT-to-AD migration available on line.
DON'T DO THIS LIVE. You need at least a couple of servers you can set up in a non-production test environment. Once you develop a plan, test in the lab TWICE!
