I recommend that you take the GMAT again for the following reason. Consider USC Marshall, which has an average GMAT score of 690, and an 80th percentile score range of 640-740. This means that your score of 640 would put you in the bottom 10th percentile of applicants. This does not mean you will not be accepted, however it is not a preferable position to be in.
There are a number of excellent GMAT resources online which can help you improve your score. MBA.com provides 2 free official GMAT tests, and we also have free GMAT advice here:http://www.stacyblackman.com/tag/gmat/
I hope this was of help.
Conrad and the Stacy Blackman
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