I've got some good news, and some bad news. The good new is, some eager chap took all the files from the Church Fathers volumes as found on sites like CCEL, and organised them according to Bible Book and Chapter, and put this all on the web. So, if you wanted to know what the Church Fathers said about Matt. 16:18, you could go to "Matthew 16" and look through all the passages from thousands of patristic texts that had referenced that chapter and book. The bad news is, I can't find the link, and after a short Google search I didn't see it either. Maybe you'd have better luck finding it, though. It really was a very nice site. Sure wish I'd found it before I spent all that time looking through notepad versions of the CCEL collection looking for info on this subject or that passage.
Try fathersofthechurch.com, the site of Mike Aquilina (RC patristics writer) ; he refers to this site periodically, I beleive.