Hi sank,
To start, a 640 is a solid score (it's just shy of the 80th percentile overall), so it could be enough to get you into your first choice Business School. Before deciding whether or not to retake the GMAT, it's important to define your timeline and goals:
1) When are you planning to apply to Business School?
2) Do you know which Schools you plan to apply to?
Since you're put in some study time already, and earned a solid GMAT score, it's possible that a classroom course is not what you actually need - you might just have to make some tactical 'adjustments' to how you handle the Test overall.
1) How long did you study for the GMAT?
2) What resources did you use?
3) What were your Quant and Verbal Scaled Scores on your Official GMAT?
GMAT assassins aren't born, they're made,
Rich