Hi there,
Thanks for posting. So there's a lot I don't know about you, since schools will look closely at your whole application and details such as your leadership experience, accomplishments, teamwork, people & project management skills, etc. But with regard to your GMAT related questions, I would say as a ROUGH rule of thumb, if you are much more than say 50 points or so off of the school's average, it could be quite tough to gain admission. So have a look at the average scores of the top 20 (on their website or rankings). You may wish to re-take if you feel it would make a big difference, since that could impact where you go... perhaps consider your practice test scores as that can give you a feel for what the difference might be, and you could always give it a shot as there's not much down side to trying. But keep us posted and good luck!