quoting a reply to a similar question in this forum:
To access your site while you are waiting for DNS to propagate, you can create a free dreamhosters.com sub-domain and set it to mirror your real domain. The procedure is outlined in the wiki article linked below;
and there seems to be another article more specific to setting up a database in advance of teh DNS change:
You can actually, as you can assign multiple hostnames to a particular database server. See the wiki article below;
for the email bit: challenge 1 is getting the emails out of powweb, whatever that is.
imagining that you got the emails out of that system in one chunk, it seems like you might be able to use PINE to bridge from an old fashioned mail file (in whcih the contents of your mail folder are all in one file) to IMAP, which can be read by the DH webmail system.
basically, you'd run PINE from the shell, config it to connect to the IMAP server, then have it explicitly access the mail file, and copy those messages all over to the IMAP folder.
i haven't actually tried this, but it seems like it could work...