The distance between RH9 and RHEL/CentOS 5 is huge, so you need to do a fresh install. Generally, upgrades from one version to the next are easy, but this is a big leap. You won't be able to just copy configuration files from /etc/, so will need to spend a bit of time configuring the new system to perform the same functions as the old one.
Users *are* relatively easy. You can transfer account settings, but this requires a sound understanding of *NIX to get right, so it may be better to recreate the accounts if you aren't particularly experienced.
Once you have matching accounts you can simply copy the contents of a user home directory between the servers, use chown and chgrp to reset the owner and group, and chmod to reset the permissions - a small shell script will save a lot of time.
If you haven't done shell scripting, this is a great way to start.
http://www.linuxcommand.org/ and an hour of your time will give you all you need.
If you are using mbox files then mail is the same principle - copy all of the files from /var/spool/mail/, and then reset the permissions on the files so that the owner is the relevant user account, and the group is the mail server account.
