Again - this is the book to read on Plato's Theaetetus and Sophist. There is also a very brilliant essay by Father Diès (who was a great Platonist) in the Budé editions:
http://www.lesbelleslettres.com/livre/?GCOI=22510100359860
http://www.lesbelleslettres.com/livre/?GCOI=22510100221030
One of the best essays on Plato's epistemology (which is as realistic as is his metaphysics: knowledge and opinion differing from one another by virtue of their OBJECTS), is in this collection:
http://www.amazon.fr/autour-platon-2-dialogues-esquisses-doctrinales/dp/B003TYLWDI/ref=sr_1_50?ie=UTF8&qid=1287545307&sr=8-50