Hi titianooo,
From your posts, it does not sound as though you put in too much study time before taking the GMAT; if that's the case, then this 640 shows that you are fairly strong critical thinker already - so your focus going forward should be on learning the necessary Tactics, patterns and little 'secrets' to the Exam. Many GMATers spend 3 months (or more) of consistent study time before they hit their 'peak' scores, so if you are not facing any immediate application deadlines, then you probably should not plan on retesting until you've put in significantly more study time.
As an aside, the 750+ score is approximately the 98th percentile - so clearly most GMATers never score that high on the Official Exam. Thankfully, NO Business School requires a score that high - so it's important to realize that the score that you "want" and the score that you "need" to get into your first-choice School are probably not the same thing. There's no harm in making this your Goal though, but you will almost certainly need a structured, guided Study Plan before you will be able to consistently score at a much higher level.
1) What study materials do you currently have access to?
2) On what dates did you take EACH of your practice CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
3) Going forward, how many hours do you think you can consistently study each week?
GMAT assassins aren't born, they're made,
Rich