Hi kuchu,
With a 640, you're actually closer to a 700+ score than you realize. However, you're likely going to have to make some adjustments to how you study and possibly learn some new tactics.
When it comes to taking your CATs, you MUST do everything in your power to make your CATs 'match up' with Test Day. That means that you must take the ENTIRE CAT (including the Essay and IR sections), away from your home, at the same time of day as your Official GMAT, under Test-like conditions, etc.
When it comes to your work itself, you have to strive to eliminate any silly/little mistakes from your work. Too many little mistakes will make a 700+ simply impossible to achieve (and while your goal is a 650+, with just a little more effort you could score higher).
How long have you been studying?
What resources have you been using?
GMAT assassins aren't born, they're made,
Rich