The problem is that you have several repos configured, and some
of them are incompatable with each-other. It's likely that
cacti is from one repo and munin is from another, and each one
requires a different version of the same package.
Try this and see if it might give a slightly different output
that is easier to decode.
Code:
yum --disablerepo=* --enablerepo=base,updates,rpmforge install munin munin-node