I believe that in your case this could easily be attributed to the shock of writing the test, the pressure more so. I haven't written my test yet but I continue to be warned daily on this site that it can certainly have that effect. If you were landing scores north of 700 on all the practice tests I think it would be worth your time to take it again. From what I've seen on the site that first experience of knowing the atmosphere and what to expect moving forward seems to be the most valuable knowledge you can gain throughout the process. Then again a 640 is nothing to shrug at, 640 is a great score! With a good application this leaves a lot of schools open to choose from. I guess what it all comes down to is where do you want to go to school. BB just wrote up a review on the advanced
MGMAT quant book, it seems like this book would be great for approaching higher level questions as well as Kaplan 800.
Wishing you the best, you should be proud of a great score, especially on the first try!
Kudos if this helps at all!