Actually your issue is a little more basic than that. You're attempting to install a nagios package which was built for centos5 on a centos 4.5 system. That's not gonna work for you. You need to use the el4 rpmforge packages, or install the el4 rpmforge-release package. Centos 4 and centos5 are not really that interchangeable because of glibc differences etc.
D'oh! I appreciate your time and will check that out.