Congratulations! That's a fine improvement and you should feel proud about your achievement! Regarding your questions, I would answer your query with more questions:
1. Are you unwilling to sit it out for another year?
2. How confident are you that you can craft a series of coherent, compelling essays and recommendations which point towards a noble end goal?
3. Would you rather pay me now or pay me later?
Here's why I ask:
1. If you are willing to wait it out for another year, why not give the GMAT another try? I think it's a consensus among users on this site that the real GMAT is harder than GMAT Prep, but if you were able to score 750 on the GMAT prep and you only got 640 on the real deal...you get where I'm going? Sure, those high and low numbers could be statistical abberations, but perhaps you could get lucky and have an abberation towards the high end on your next real GMAT.
Moreover, if your work schedule and frequent battles with illness forced you into an irregular study schedule, do you think your performance would spike with more regular study?
2. For each respective b-school class, the Adcom seeks to develop a "family" with members from all corners of the globe, from all professions and with wildly varrying career goals. If you are able to coherently articulate your story through the essays and your recommendations, then you may have a chance of getting into one of your top schools. If you plan to apply to US business schools, you have ethnic diversity in your favor, but what about your career goals? Do you want to be another private equity pro or do you want to combine a rigorous quant curriculum with intense focus on a broad set of leadership skills so that you can manage a network of primary schools in your homeland? The idea which I am trying to convey is that even if your GMAT score is below the 80% range of your dream schools, you still have a decent shot of gaining acceptance if your application does an excellent job of selling your unique story.
3. Not that money is the be all - end all, but for many aspiring MBA's, it is certainly a major factor in their decision to enter b-school. If this is the case for you, it may be worth sitting out one more year to bump your GMAT by another 40 pts or so. If you do some googling for starting MBA salaries compared to average entrance GMAT scores, you will quickly see the monetary reward associated with that extra 40 pts on the GMAT. Plus, you may also find during the course of your googling that many employers are now using the GMAT to sift through prospective MBA hires. How would you feel if you did get into your dream school, but upon graduation, your dream employer dinged you just because of your GMAT score from three years ago?
All that being said, congratulations on a terrific improvement. Kudos!
My GMAT Debrief: http://gmatclub.com/forum/my-gmat-experience-540-to-92850.html
Every man I meet is my superior in some way. In that, I learn of him. - Emerson