Depends on the load the box will see. Typically, most 'shared' hosting sites will have all the services on a single box, and the load is manageable. The nice thing about RC's configuration is that, if you needed to quickly move the MySQL database off the server, you could do it with relative ease, just exporting the DB and changing the hostname, and it's done. Same with the IMAP server, etc. I'd say, start with a single box, and grow out from there. You might find that consolidated services allow for better performance when the number of users is low. Once it starts growing, just be mindful and keep an eye on the box.