1) I would look into Horde/IMP (http://www.horde.org/
) for your webmail needs. This works best with an IMAP backend like uw-imapd, which comes with redhat. I recommend IMAP any day over pop3. That way you can access ALL of your email from anywhere using the webmail client (horde/imp). However, you can still use the uw-ipop3d pop3 server that comes with redhat.
2) I think rh9 is far better than 8. I know 7.3 is a very stable server OS, IMHO.
3) I havent played with this much. There are some theories. Check out the samba lists for more information on this specifically.