I am not sure what those schools require, but for top programs, you would either be disqualified because of your GPA or need a very high GMAT to make up for it. I would not advise you to target any score. Just study as hard and much as you can and do well. If a school requires a 650, for example, scoring a 700 would not hurt, only help. On the other hand, if you only study enough for a 650, you may very well score below that.
I would not leave your job. What if you quit your job, score poorly on the GMAT, and do not get into any of your target programs? That is a pretty big risk to take.
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