Thanks Rich.

I am a full time working professional, so can devote upto 2 hours on a week-day and 4-5 hours on weekends. Will that suffice to score 650+, as I plan to apply to Top 50 American Universities ?

We'd have to know what was causing you to score 270, but in my experience the further you start from the center, the slower you tend to progress. (If you start low, it's hard to get to average/above average, and if you start high, it's hard to get to super high.) It's possible, but at the low end of the curve that's often a language barrier that itself takes a lot of time to break through, and it's really really hard (and totally unfair!) on a student to have to learn difficult English AND the GMAT at the same time.

Three months doesn't seem sufficient to me (though it might well be) but in any case you really want to think in terms of hours, not weeks or months. Start with 20+ hours a week of study and see how much you're able to progress in a month, and that will give you a strong idea of exactly what lies ahead of you.