Note 1: No two systems are exact, so these are general terms, and not intended to be exact specs.
Note 2: The GW domain is neither like a MS domain, nor an Internet domain name. It is simply the name of your GW domain.
Note 3: Be careful when creating your new Software Distribution directory. You will want the new SDD to be located on the new box.
You do not want to create a NEW system. You want to create a new Domain in your existing system. As long as you are UPGRADING to a new box, you will not effect your existing server. Once you have the new domain, add an MTA and Post Office. You can actually build a running Domain and create a few test users without effecting the old domain. You should then be able to send email between users of each domain.
When installing, you WILL point to your old GW system, and it is a bit scary, but if you read the screens and keep in mind that you are adding a domain to the existing system, you will begin to see what the screens are asking for.
Once you are convinced that the new domain is working, you can copy users over a few at a time. After all of your users are copied to the new domain, you can easily delete the old domain from eDir.
Hope this helps.