Hi Navski,
While the most realistic CATs available are the 6 from GMAC, retaking a CAT that you've already taken is NOT a realistic way to assess your skills - since you will likely see some 'repeat' questions that you have already faced. Unfortunately, seeing even a couple of repeats can 'throw off' the Scoring Algorithm and impact your pacing, energy levels, fatigue, etc. (meaning that they would likely all appear to be better than they actually are). By extension, the results from your 2nd attempt at GMAC CATs #5 and #6 are likely 'inflated.' Thankfully, the CATs from Kaplan,
MGMAT and Veritas are all 'close enough' to the real thing that they will provide you with a relatively realistic score assessment (assuming that you correctly take the CAT in a realistic fashion) - so you can use any NEW CATs from any of those sources next.
If you don't have a specific Score Goal in mind, then it's tough to tailor specific advice for your situation. For example, raising your recent 640 to a 650 is a notably different task from raising the 640 to a 700 (or even higher). Thus, if you have an idea of the type of Score that you might 'need' for your applications, it's important to define that Score as soon as possible so that we can make sure that you're making the necessary adjustments to your studies.
GMAT assassins aren't born, they're made,
Rich