Need to improve GMAT quick

Hello all,
Currently I am waitlisted at Duke, and I think that improving my GMAT score would significantly improve my chances of getting in. If it doesn't it will help me out for the next year when I apply to other MBA programs. So I have taken the test twice, the first time I score a 650 (49Q, 29V). The second time around I improved very slightly to a 670 (48Q, 34V). I am in the 80% range but the problem is my GPA is pretty low as well (3.0). I need a >700 score to offset my GPA so that there are really no questions about my candidature.

Now I have tried studying on my own but I think that is not working out properly. On my own I think I could get a >700 but it would take time that I do not have so I have decided to look into taking a course. A short, intensive course. And i wanted to get the opinions of those who have either taken a GMAT course or even private tutoring to give me some direction. Which course (preferably short intensive courses) are good and provide someone like me (on the cusp of 700) that extra umph to break the 700 barrier? Veritas, Kaplan, Princeton Review, Powerscore? I looked around for a sticky which might talk about the various courses on GMATCLUB and couldn't find it. Any info is GREATLY appreciated.

Thanks