Short answer, you could try "up2date --undo".
*It could be usefull to find out *why* metacity won't load. For that you'll have to manually find out what metacity depends on (GTK, Pango, etc etc) check the rpms and those of the dependencies, and check which files have been changed.
To verify a package: "rpmverify <packagename>"
To find a package's dependencies: "rpm -qR <packagename>" .