I would stay away from the "brutal" questions, and really work on building a foundations and then timing. Make sure you know the ins and the outs of the entire tests, and all question types. Make sure that you can answer ALL easy and medium level questions, with some knowledge of the advanced topics. I would take the GMAT prep test #1 from mba.com when you start your studies, and #2 right near the end. The 1st test will tell you how far you have to go, and the second will tell you if you are on target. Both tests are the best estimate of what you will get on test day. Remember, the verbal will really help drive your overall score, so if you are solely going for a 650 and dont care much about the quant, you should focus your time there.
You look like you are close to your target score, but make sure to give the GMAT you full attention or it will come up and bite you. Considering how well you are scoring with so little prep, I wonder why you are not pushing for a higher score?
Good luck