Well, a 640 is pretty good in the general scheme of things but you should do better. What was the score breakdown? Provided that you have enough time in the next month to keep up with your studying then a re-take in 30 days seems like a good idea. A few things:
1. Have you only taken the GMATPrep twice? What was the breakdown?
2. Your idea is correct that you should spend the month on solidifying your position and test strategy but I would do a lot of timed sections. If you haven't done the 9 GMAT Paper tests I would do all of those in the next month. Only do the verbal section. Give yourself 80 minutes and do all of the sections together in one sitting. If you haven't done questions pack one I would work through the difficult and medium verbal and quant. Also, the ten FLTs are pretty good verbal practice for SC and CR.
3. For Quant I would take Manhattan GMAT
and GMATClub CATs every other day. Make sure to leave time to analyze wrong answers.
Also, if you have an error log go through it and re-visit all of your weak areas.
You must be tired of studying BUT look at it this way: you are extremely close to your goal and with just a little bit more organized work you will get there.
Try your best to enjoy studying and to keep it all in perspective. Make sure to build in plenty of fun activities in the next month
to give yourself space to relax. Make sure to work in plenty of time away from thinking about the test each day and to give yourself complete days off when you need them. Let me know if you have any questions on this.
PS: Completely optional but it might not hurt to work in some GRE reading comprehension just to give yourself some extra challenge to build up your confidence. One long and one short passage every other day. 1:45 per question.
"It is a curious property of research activity that after the problem has been solved the solution seems obvious. This is true not only for those who have not previously been acquainted with the problem, but also for those who have worked over it for years." -Dr. Edwin Land
GMAT vs GRE Comparison
If you found my post useful KUDOS are much appreciated.
IMPROVE YOUR READING COMPREHENSION with the ECONOMIST READING COMPREHENSION CHALLENGE:
Here is the first set along with some strategies for approaching this work: http://gmatclub.com/forum/the-economist-reading-comprehension-challenge-151479.html