Phil over at hyperekperissou asked a couple of questions about the state of patrology online. You’ll find some good discussion in the comments on the post. I found some PDF scans of PG that I didn’t know about (see here for Irenaeus). He then was kind enough to summarise the big picture status of things in this post: Patrology Online – Wrapup.
See here for some discussion of electronic versions of PG.
p.s. You’ll see that I have most of the major sights on my sidebar under Patrology Resources as well as some in my post on Patristic Biblical Citations.