Karen...In addition to the aforementioned cruise lines, you might also check out Oceana Cruises. They took over some of Renaissance's ships after the bankruptcy, and they sail the Med between Spain & Greece. The ships are luxurious and carry less than 800 pax. We've been getting brochures from Oceana, since we were Renaissance customers, and their prices look pretty good. Also, they're presently offering free or reduced roundtrip airfare from the U.S.
As for ports of call, we especially liked
Barcelona, Palma de Mallorca, Marseilles, Villefranche, Ajaccio (Corsica) and Naples. Alas, we never did get to Venice (but we will someday). Go for it, and happy Med cruisin'!