Hi Queen199212,
First off, a 640/Q45 is a solid score (it's right around the 70th percentile overall), so it could be enough to get you into your first-choice School. There's no harm in retesting though, and you have the potential to pick up some serious points in both the Quant and Verbal sections.
Before I can offer you any advice on how you might proceed with this next phase of your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied?
2) What study materials have you used so far?
3) How have you scored on EACH of your CATs/mocks (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