Get an MBA if you have an idea of the sort of work you want to do after graduating. Most people getting their MBA's (esp. Rotman) have a laser-focus on what they want to break into after, so they're far more competitive and marketable. My advice is, first off, not to rush this year's class.
Spend a few months understanding the options available to you post-MBA given your legal and HBA background. Call up a few friends or contacts in various industries and get their opinion on the type of work that they do. This will also help in establishing fresh networks when you're recruiting post-MBA. An MBA is not like the HBA; there's a level of concerted, premeditated network-building during your MBA that yields 100x more leverage than drunken Thursday nights at the Frog, since everyone getting their MBA knows 100x more people in their respective industries, than they did while doing their HBA. If you don't know what you want out of your MBA, you'll miss out on this value greatly.
640 is not a bad starting score, but 700+ is money. Spend two hours a day for two months studying and you'll hit 700+ no problem. Buy the full OG set and do all the questions online - aim for 30Q's and 30V's everyday (2 hours). Also get the Manhattan SC book and study it religiously. Spend the last two weeks before the exam doing 3-5 practice exams (one every two days). You'll be good to go.