Hi niteshwaghray,
First off, a 640/Q45 is a strong score (it's right around 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:
ask-admission-consultants-124/GMAC has publicly stated that the Official Score that you earn on Test Day is within +/- 30 points of actual ability. Assuming a similar 'swing' in how your CATs function, your various score results aren't as vastly different as they might appear. Your score 'range' is likely in the high-600s (with a high around 700 and a low around 640). That having been said, there are a variety of factors that could have impacted your performance on Test Day - the two likely "causes" involve either something that was unrealistic during practice or something that was surprising (or not accounted for) on Test Day. If you can answer a few questions, then we should be able to figure this out:
When you took your CATs:
1) Did you take the ENTIRE CAT each time (including the Essay and IR sections)?
2) Did you take them at home?
3) Did you take them at the same time of day as your Official GMAT?
4) Did you ever do ANYTHING during your CATs that you couldn't do on Test Day (pause the CAT, skip sections, take longer breaks, etc.)?
5) Did you ever take a CAT more than once? Had you seen any of the questions BEFORE?
Having to guess on the last 4-5 questions in each section could very easily have hurt your scores (especially if you got all of those questions wrong). Any questions that are left unanswered are 'penalized' (and that penalty is worse than just simply getting the question wrong). However, leaving just one question unanswered would only have impacted your Verbal score a little (if at all, 0 - 10 points off your overall score) - and if you actually bubbled one of the answers, then you were credited with answering the question (so you would not have been penalized).
1) What is your goal score?
2) When are you planning to apply to Business School?
3) What Schools are you planning to apply to?
4) Have you considered purchasing the ESR for this Exam? While the ESR doesn't provide much information, the data might help to point out some areas that you need to work on to score higher.
GMAT assassins aren't born, they're made,
Rich