A simpler approach would be look for separate how-to's on
- centos (help you install the OS)
- postfix, sa and clam (as long as the how-to is written for Redhat
you should be fine) 3. dovecot
since these are somewhat separate topics.
also take a look at http://www.zimbra.com
It's Postfix based, includes spamassassin, clamav, imap... They have a free version, that works very well and handles heavy volume very well, 30,000+ emails/day.
I agree that Zimbra is great idea, but I am guessing that the original poster is new to Linux and I think that is it important to learn more about how Linux works before putting something like Zimbra online. Putting some time into learning the nuts and bolt of Linux and email administration will pay off in the long run and make the transition to Zimbra easier for the poster and safer for everyone else.
Ed Bailey