The field is broad, and if you can get your GMAT up to the 630 range, you would have a shot at a school in the top tier, say with a ranking in the 10-25 range, which I would recommend. Best bet for you is to do some research on which schools have a 630 within their 80% range--Bloomberg Business Week has a very robust website where you can actually pull data from schools to see how many admitted applicants at various GMAT levels. This will help guide you and give you your odds. GMAT is not the only factor of course, but it is an important one, since it's the only truly objective way for schools to compare applicants with each other.
_________________
Bryant Michaels
Admissions Consultant