I lived in Philadelphia for a few years. I really liked the Queen's Village and Old City Area. If you can afford to spend a little more $$ Old City is nice. The only thing is that parking is a pain. But if you're on a graduate student's budget, I'd live in Queen's Village. It's a few blocks south of South Street and its safe, quiet, and very cute. Plus, it's more residential than living right in center city. There are lots of good restaurants as well. And there are buses that will take you to Penn, Drexel, or Penn within 20-30 minutes.