I will recommend HowToForge as having good step-by-step instructions: http://howtoforge.com/virtual-users-domains-postfix-courier-mysql-squirrelmail-centos-5.3-x86_64
I typically use this setup for authentication: http://wiki.dovecot.org/HowTo/PostfixAndDovecotSASL
And this site provides a good overview: http://rimuhosting.com/support/settingupemail.jsp?mta=postfix
Postfix virtual has some good ideas on database design for virtual users: http://www.postfixvirtual.net/postfixconf.html
A few notes:
A nice setup is Dovecot and Postfix - using Postfix for delivery, and Dovecot for authentication and IMAP. In this way you can easily create virtual users (for instance, in MySQL) and don't need to bother with modifying files for each new user/domain. (On the other hand, a pam or flat-file approach might offer slightly better performance).
I would highly recommend Postfix-admin (http://sourceforge.net/projects/postfixadmin/ ) for use to manage your virtual domains/mail-users
Postfix supports redirecting mail - so your MX record could point to your postfix server, and if you have a redirect setup for that particular address, postfix will deliver the mail to the desired address (either local, or as in your example, GMail). Again, with PFAdmin, you can configure the redirects in a database.
You can setup an MX record pointing at GMail (http://www.google.com/support/a/bin/answer.py?answer=33352) which would essentially result in all mail coming to your server ending up at Gmail; but you would still have control over the mail you send from your server (alternatively, you could just use Gmail as your SMTP server as well).
To improve deliverability, I would definitely suggest adding an SPF record to your domain, as well as implementing DKIM/DomainKeys signing (via dkimproxy).
As a webmail interface, SquirrelMail has a small footprint; however RoundCube is more 'modern' - both work well.
Also, run ImapProxy if you will be using IMAP as it greatly reduces server load.
A side note - the configuration files for Dovecot changed for version 2 and most guides reference the 1.x series. Check your version if you use Dovecot.
There are also some control panels (e.g. Webmin/ISPConfig) that will help to configure your server - I would not recommend them though unless you are comfortable enough to undo anything they do.