For most people the score on the GMAT is not a reflection of their skills or intelligence, but of the number of practice exams they have done. So try buy/borrow/download/whatever as many practice exams you can find - the newer the better as they keep on changing the format of the test and today's test might look different from the ones they had in the Nineties...
In terms of MBA yes or no, I would spend do a thorough analysis of what you want to do as a career and then find the school that has the program that offers the most suitable program.
The MBA programs have evolved a lot and it is not a one size fits all - there are programs directed at people who want to start their own business, at people who want to work with numbers, at people who want to work with people, at people who want more of a marketing angle, at people who want to be problem solvers - you name it, some school will offer it.
So the best strategy is to have or develop a clear idea of what you want and then apply with the schools that can help you achieve this. It also increases your chances of admittance a lot, if your application letters and essays show them what your thought process was in selecting their school.
Also look at other graduate programs. Maybe a Masters in Financial Mathematics or Masters in Finance could be better suited than a MBA if you want to go into that field.
The corporate environment has changed. Companies are not looking for as many generalists as they used to, but they always need specialists.
