Hi FreakingGPAforZero,
A 650 is a strong score (it's right around the 80th percentile overall), so it could be enough to get you into your first-choice School. As such, a retest might not be necessary. You would probably be able to better assess the strength of your GMAT score if you research a bit into the Schools that you plan to apply to. Many Schools provide information on their respective class 'profiles' (including average test scores, etc.) - if the School's website doesn't provide you with the information that you're looking for, then you could contact the School's Admissions Office directly to ask your questions.
All of that having been said, from what you described, you limited your studies to certain subjects - which means that you left a lot of potential points 'up in the air' because you didn't hone your skills in those other areas. If you do choose to retest, then you could potentially pick up some big points if you broaden your focus and put in the necessary effort.
GMAT assassins aren't born, they're made,
Rich