I would recommend Princeton Review. The materials are different than the ones you can get in the bookstore, and the instructors are really cool - they will help you as much as they can for free outside of class. Plus the classes are really small.
It's impossible to say whether you need a class to get to 770 or whether you will get there after doing the OG, because you haven't shared any practice test scores or previous results. A 770 is an extremely high score that very, very few people get. You could take the best prep class in the world and do every practice test out there and not get 770. Why such a high target score?