I just used that howto as well on my system, since I may be scaling up the number of mail users on my newly built server.

Defilitly run Postfix over Sendmail. IMAP wise, I have no experience with Cyrus, but have run Courier with no issues on multiple machines. In any case, use Maildir with IMAP along with a filesystem built to ahve tons of tiny files on it like ReiserFS. The idea of mbox IMAP just scares me about as bad as massive e-mail archives in Outlook .pst files.