Hi Addictedk89,
A 640/Q44 is a solid performance (it's just shy of the 80th percentile overall), so you've done pretty well. While this set of scores would be at the lower end of the "range" for your target schools, this score would not take you out of consideration at any of them (it just wouldn't be a "plus" in your application). You're looking at schools that are going to reject 90% of the applicants who apply; it's worth noting that almost all of those applicants are going to be great applicants too. This is all meant to say that your OVERALL application has to be strong. If you don't have that many amazing aspects to your application, then you might want to put in more time on your GMAT studies so that you can make your GMAT score a "plus" - this might also mean that you have to push back your application to a later Round.
Most Test Takers spend 3 months (or more) on their studies, so it sounds like you might not have done enough work yet to earn a higher score. Another issue seems to be that you self-taught your way through a series of books. You've come up with a fairly consistent way of doing things, which is why your scores stayed in the low-600s. To score at a higher level, you're likely going to need consistent, professional guidance over your next round of studies. To that end, there are plenty of options to choose from; there are Guided Self-Study Courses as well as Instructor-Led Courses. Most GMAT companies offer some type of free resources (Trial Accounts, practice problems, etc.) so that you can try out a product before you buy it. We offer a free Trial Account and I know that both
MGMAT and Veritas offer free resources on their respective websites. You should take a look at the various options, then choose the one that best matches your personality, timeline and budget.
If you have any additional questions, then you can feel free to contact me directly.
GMAT assassins aren't born, they're made,
Rich