To be honest, I think your undergraduate GPA might be a big problem. Even an incredibly high GMAT score (above 700) might not be able to make up for this "weakness" of your application. The graduate GPA of 3.8 is pretty much "average" for applicants I would say.
I think very few people write an undergraduate thesis so don't worry about that, but you should try to get your hands on some research experience.
If I were you I would not aim at Top 20 programs. I don't think that's a realistic goal with your profile right now. Unfortunately you can't improve your undergraduate GPA, but you can definitely work on other parts of your application.