Hi jenks88,
First off, a 640 is a strong score (it's a bit shy of the 80th percentile overall), so it could be enough to get you into your first-choice School. As such, a retest might not be necessary. Depending on the Schools that you plan to apply to, you would likely find it beneficial to speak with an Admissions Expert about your overall profile. There's a Forum full of them here:
https://gmatclub.com/forum/ask-admission ... tants-124/If you're certain that you want to retest, then it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long did you study before each of your GMATs?
2) How did you scored on each of your GMATs (including the Quant and Verbal Scaled Scores for each)?
Goals:
3) What is your goal score?
4) When are you planning to apply to Business School?
5) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich