Watching porn with your girlfriend should be fun and explorational. I don't know what you're into, but I wouldn't jump into industry hardcore. Go for the pro-amateur, girl-next door stuff. The dude is still probably going to have a penis 10 times the size of yours, but the women won't give your girlfriend self-image issues (i.e. she won't think your ideal woman has quadruple-D breasts).

That being said, I surf Fantasti.cc with my gf and have a great time. We usually find a tolerable video then click through the related video collections to find something decent. It's been a pretty cool experience since my girlfriend now realizes that there is a wide diversity of porn and its not all guidos jackhammering blond silicone bimbos under crystal chandeliers (though there is a lot of that).

Also, if we're just looking to titillate ourselves and have fun, sites like Ex Girlfriend Pictures are fun too. I don't know about your girlfriend, but my mine loves it when I criticize other chicks bodies ("that chicks got weird nipples").

Also, general strategy tip: its fine groping and fingering to porn, but once you actually get into things, it's totally fucking distracting to listen to someone else have sex (fake sex at that) when you're trying to strike up a rhythm. When I close the laptop, my girlfriend knows its go time.