The only time you might be missing RPM's is is you did a minimal install. The same thing with any RedHat distro. CentOS 4.1 is the same thing as RedHat EL4, the only difference is that CentOS removed all the references to RedHat, changed the update mechanism, and used their own graphics.
I personally do a server install, then customize it so it only installs the RPM's that I need. The packages you really need to pay attention to are the developement ones ( contain -devel in the name). Without those, you won't be able to do the customapache build. The things you don't really need are X and X applications (mozilla, gnome, kde, ...).