Hi!
I can tell you that, a few years ago, with exactly one night of studying, I scored a 690. However, I feel I was lucky to do so, and I know a good deal of intelligent people who scored as low as 550 at first glance, only to surpass the 700 mark later. I may have been at an advantage, seeing as I was still in school and taking a lot of math-heavy classes, as well as a lot of literature/philosophy courses.
A few years later, I decided to put three months into studying, and wound up with a 740, mainly due to better preparation with regard to the GMAT-style questions.
To be perfectly honest with you, a 650 is a very good score if you didn't study at all. Unless it was a total fluke, I would expect you to score 700+ on the real thing, if you take the time to familiarize yourself with the GMAT as an exam and the attributes that it tests. Hitting 700 with no prep is rare, because the GMAT is not a test that measures intelligence, but rather preparation, stamina, and resilience.
Personally, I would advise you to pick up the Official Guide, 13th ed, and do that diagnostic, too. From there, focus on your main weaknesses, making sure you take enough time to brush up on your strengths, as well. If you want to score 700+, you should get to a point where you're able to demolish 85-95% of the questions on OG13.
Question for you - what was the split of your 650, in terms of Q and V? That could tell you a lot about the direction you should take from hereon out.