Opinions are mixed on this issue, both from the consulting side and from admissions conversations I've had or seen. But it sounds like you've already strengthened your stats knowledge and gotten the grades to support that, so calculus would be next on your list. You have multiple goals in taking graded courses like these: first, of course, to provide an alternative to low undergraduate grades. But second is to gain skills you know that you will really need as an MBA student -- the more sincere you are about that, the better it will work out. A third benefit that I've noticed is that people who take serious graded courses do better in the quant section of the GMAT as well. So whichever direction you choose, you're doing the right thing by taking this on.

Good luck!
Margaret Strother