Your GMAT score is only one aspect of your MBA application and I must say that 640 is not a bad score depending on which schools you are aiming at. For the 20 top schools the average varies from 650 to 750.
Before investing in any of the commercial GMAT preparation material, you may want to consider the following options
1. Free online material available
2. Social learning sites like Grockit
3. Official GMAT book, The official guide for GMAT
In addition to strong academics, B-schools are looking for you to demonstrate your personal attributes, leadership skills, career goals and creative thinking in your applications.
I will also encourage you plan your MBA better and spend sufficient time on targeting schools, understanding their requirements and most importantly assess your post MBA options. Knowing why you want to do an MBA is generally a good starting point.
Good luck !!
Maria
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