please read the centos wiki
and use the groupinstall
yum groupinstall "Development Tools" "Development Libraries"
and double check the names from the output od
also you have " rpmforge" installed on CentOS 5 / RHEL 5
rpmforge is KNOWN !!!!!!!!!!
to have many ,many , many incompatible rpms that are incompatible with the redhat/cent base and update repos
and you do not have "yum-priorities " installed and configured
and heed the warnings
the yum plugin
this was "? mostly ?" fixed in CentOS 6
Also have you cleaned the database
yum clean all