Hi Ramesh,
Many Test Takers are unhappy with their first Diagnostic CAT scores, so you're not alone. It's important to remember that the GMAT is a consistent, predictable Exam, so you CAN train to score at a higher level. With a 640 score goal, you'll have to make some big improvements to how you handle BOTH the Quant and Verbal sections, so you have to make sure that you've given yourself enough time to learn the content and the proper Tactics.
I'd like to know a bit more about your timeline and goals:
1) When are you planning to take the GMAT?
2) When are you planning to apply to Business School?
3) Your post implies that you're interested in just 1 School; what School is it?
4) How do you know that you "need" a 640?
GMAT assassins aren't born, they're made,
Rich