Have a look at this tutorial: http://www.howtoforge.com/perfect_setup_fedora_core_4
In the tutorial I install the package imap; leave it out as you have installed Dovecot. You should then add these steps after the Postfix configuration because Dovecot uses Maildir:
postconf -e 'home_mailbox = Maildir/'
postconf -e 'mailbox_command ='
If you want to install ISPConfig afterwards, you can create your mailboxes with the ISPConfig web interface; if you want to create your mailboxes manually, have a look here: http://www.howtoforge.com/forums/showthread.php?t=2