I think you should focus on GMAT and crack it. 640 is very low. If you can get a better score, that will help your case much more than the supplemental course.
Don't take any supplemental course if it is not from a top school. Nowadays, there are millions of sites offering such courses. I think they dont help you much. You are going to a business school to learn those skills. If you applying for fall 2013, I suggest you focus on your essays. They matter a lot. Good luck!www.InterviewBay.com
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