Hi Omha,
From your post, it's not clear how long you've been studying. At face value though, raising a 300 to a 650+ would likely take at least another 3 months of consistent, guided study - and you would have to make significant improvements to how you handle BOTH the Quant and Verbal sections. Before I can offer you the specific advice that you’re looking for, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied?
2) What study materials have you used so far?
Goals:
3) When are you planning to apply to Business School?
4) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich