The National Security Agency publishes a great collection of documents that provide detailed security configuration guidelines. The full index can be found here:
http://www.nsa.gov/ia/guidance/security_configuration_guides/index.shtml
We particularly like the guides for RedHat EL5:
“Hardening Tips For Default Installation of Red Hat Enterprise Linux 5”
http://www.nsa.gov/ia/_files/factsheets/rhel5-pamphlet-i731.pdf
“Guide to the Secure Configuration of Red Hat Enterprise Linux 5”
http://www.nsa.gov/ia/_files/os/redhat/rhel5-guide-i731.pdf