redhat-config-packages: This is the primary mode of operation. It is
used to install extra packages from package groups included in Red Hat
Linux. This uses CDs to install the packages by default. You can
instead use a Red Hat Linux installation tree with the option
'-tree=/path/to/tree' or use a directory with Red Hat Linux ISO images
with the option '--isodir=/path/to/dir'.
Of course, using yum may be a better idea as it's better with updates and dependencies, but I didn't find a list of the installable packages that system-config-packages gives you...