Overall, your background sound pretty unique, especially given your international experience and communty involvement. Often we hear from students who have great GMAT scores but unremarkable backgrounds, but your profile sort of sounds like the opposite of that.
It's not impossible to get into those schools with a 640, but it will be tough. What concerns me more is that you mention that you didn't have a lot of time to prepare for the GMAT. At the risk of sounding like I'm preaching, you probably should take the exam more seriously, and consider re-taking it only after you've had time to prepare enough that you believe your score will improve. What I recommend against is taking it again right now, without any new preparation, and just hoping for a better score. Since you didn't study very much, you probably can improve your score by at least 30-40 points if you just devote some significant time to the exam over the next few weeks.
If you're trying to also write your essays, though, then it will be tough. The risk is that adcomms will see your GMAT score and your essays and recognize that you rushed through your application. If they think that, then your odds of success will be small.
I hate to sound so negative! But I recommend that you slow down and try to improve your GMAT. The GMAT isn't the most important thing, and you *can* get in with a 640, but I worry that you're just rushing through your app, and the overall product may suffer.
Veritas Prep | GMAT Prep | MBA Admissions Consulting | Co-author, Your MBA Game Plan
Save $100 on Veritas Prep GMAT Courses And Admissions Consulting
For a limited time, receive access to five Veritas Prep Computer Adaptive practice tests when you purchase a Veritas Prep GMAT book! Buy Now!
Enroll now. Pay later. Take advantage of Veritas Prep's flexible payment plan options.
Veritas Prep Reviews