Ok, well first thing, your Exchange 2007 server must be at SP2 level, to be able to move your mailboxes to the new server. But, before you do anything here I would ensure you have a full, recent backup of all your Exchange data.
Once that's done, the first thing I would do is get your Exchange 2010 server installed in your organisation and mail flowing between servers, so you can be sure that when you start moving mailboxes, any mail that comes in will end up still getting to any mailboxes that have been moved, and internal mailflow is working ok. I would setup a test mailbox on the new server and make sure everything is working as it should.
Once your happy with that you can begin moving mailboxes over to the new server. Bear in mind the move must be done from the 2010 server (see here). This is an on-line move, so users should not be affected.
One caveat I can think of with Exchange 2010,its got rid of WebDav, so any Entourage clients will need to download the newer EWS capable version of Entourage.
There is an excellent article here, that gives more detail on the steps for doing this than I have, definitely worth a read.