[CentOS] How to install the devsys after the fact

I'm running with CentOS 4.3 and the system was setup originaly with no
development system and no kernel sources. I'm trying to get the headers and
libraries installed and it is giving me fits. I've followed the suggestions
on several threads and tried to yum install gcc, kernel-headers,
kernel-devel but I seem to not be able to get /usr/include or
use/include/sys packages... don't know what they're called even.

Has anyone succesfully installed these things after the OS and can you
please point me at some howtos or docs on how it's done?

Any help gratefuly appreciated.

Cheers!

--p