Hi dreamyagrawal,
To start, your 640 and 600 are closer than you probably realize. GMAC has publicly stated that the Official Score that you earn on Test Day is within +/- 30 points of actual ability. By extension, your 2 Official results show that you essentially performed the same each time (about 620 +/- a few points). If your goal is to score significantly higher than that, then your immediate focus right now should NOT be on taking lots of practice CATs - it should be on honing the necessary skills to score higher on the GMAT. While you would certainly need a new 'block' of CATs to work with, you will also need some new practice materials that will train you in the necessary Tactics.
Before I can offer you the specific advice that you’re looking for, 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 attempts at the GMAT?
2) What study materials have you used so far?
3) How have you scored on each of your CATs (including the Quant and Verbal Scaled Scores for each)?
Goals:
4) What is your goal score?
5) When are you planning to apply to Business School?
6) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich