Exchange upgrade-move on new server

I have just bought a new server to replace our old Exchange 5.5 server, I plan to upgrade to Exchange 2003 at the same time and wondered which was the best method of moving users mailboxes from one server to the other but keep all messages and email addresses.

I have a windows 2000 server network using a sinlge domain and need to setup the new server with the new one in place (ideally) then move mailboxes from one server to the other (perhaps over night). Is this possible or do I have to take the current server off line and then call the new server the same name and do a restore? As if I had a bad crash on the exchange server.

Any ideas, links and tips very welcome.

Thank you