while a 680 is a respectable score, if that was your first time then the odds of you scoring higher is positive.
You didn't give any background information about what you want to do post-mba.. why you want an mba... etc etc.. all the really important things.
"Maybe I should try to get into a consulting firm and work for a bit, retake my GMAT and then apply to US B schools" That doesn't sound like a bad plan for someone without work experience. The lack of work experience, not your GMAT score, would be what would be a barrier to entry to a top business school. Folks w/o work experience are rare (it does happen) in business school. The good thing, however, is that you're young and are already thinking about this stuff. You probably don't realize it yet but you're waaaaaaaaaaaay ahead of the game if you decided to apply in a couple of years.
The other thing that I will say is this... don't wait a couple of years to re-take the GMAT. Take it THIS YEAR if you can and get it out of the way. You're brain is already in study mode and you don't want to lose that momentum. As the years go by it will be tougher and tougher to A) want to study and B) to have the mental preparedness required for a 700+ score. It's definitely doable but if you take it now you will thank yourself in the future.
Find out more about my MBA ADMISSIONS BOOK on how to navigate the b-school application process: http://bit.ly/1a9U7Cl
Blogged my way through the application process, 1st & 2nd year of school at Johnson at Cornell University - Graduated 5/26/2013 - Follow my MBA journey here @ http://www.money9111.blogspot.com | 1 New Post on 10/10 !