OS Installs

Linux

If you're moving an existing system to new hardware, or installing as an upgrade, you need to make it act like the old box

use rsync or whatever to make a copy of the original hosts /etc, /root, /usr/local and possibly /home. These should be copies used as a reference, not used to overwrite the new host's /etc/, /root/, /usr/local etc...
if this is a part of a HA cluster, set up rsync copies.
install obvious software (web server? NTP?)
configure new nagios host for new host with same tests as original
make it pass those tests
if there is still something visibly wrong, add a test to nagios
sync up perl modules - get autobundle from original parse a little by machine, but mostly do this by hand.
sync up RPMS - Again, don't just blindly install everything.