Transferring a domain

This will be the second time I have done this in two years, so I don’t understand why I am so confused. Anyway, I thought I would post here instead of ask support because I’m sure others can learn from this.

I have recently signed up for an account with Dreamhost which allows three domain names. The first domain name is a new one I get as part of my new account. The other two already exist and are hosted elsewhere. I intend to migrate them to Dreamhost once I get them mirrored here.

That’s the essence of my question: how do I mirror those sites here? If I FTP using the usernames I set up for these other domains, I am dropped into a symbolic root directory. Presumably I should put web documents in a subdirectory, but what should i name it? Should I use the domain name as is the case with my first domain? In that case, what is the URL I use to check the mirror before I transfer my domains?

The bigger issue concerns e-mail and databases. How do I ensure that I receive all the e-mail destined for one of these domains during the switchover? I know I will want to set up mailboxes with my dreamhost account, but how do I link them to the new domain?

Fortunately, my old accounts don’t have any live databases (just tests), but I was still interested in setting up PostNuke here and having it ready to go when the switchover occurs. I am not sure where to install it, however, or what I should enter as the database hostname. I suppose I can set it up on my first domain and then move it over in the dead of night after my domain changes have been propogated, but I am wondering if there is an easier way.