California allows 0% on the front passenger windows. I did for my back window, 30%, rear passenger windows, 40%, and front passenger windows 50%. Supposedly, California cops don't really pull you over if they can easily see into your car, so its not just about how dark your tint is, but also how clear.
I just checked out Penn state laws and it looks as though there are ways to get around law:
http://www.berksweb.com/users/dmv/tint/
I slapped down some dough and got high quality install job and high end tint (Huper Optik) so its VERY clear, yet still does the job and looks great (see picture below):
Depending on your interior color (mine is grey), the tint will look darker or not as dark. Because of my light interior, it looks a little lighter then someone who has black leather seats. I highly reccomend finding a good tint store in your area (which might take some time) and see what the consensus is. The tint installers will know best. A good place to find tint shops:
http://www.tintdude.com/Pennsylvania.html
-Stink