I think the amount of math that will be required of you in a PhD program can be pretty formidable... more than most people expect, so taking additional courses is a good idea.
From what I can glean, most schools are looking for you to have taken Calc I & II as well as linear algebra (which has Calc III as a pre req). These classes will form the basis for probability theory and thus inferential statistics, which you will use throughout your career as a researcher. None of these classes will help you on the GMAT per se, but if you can get through all of them you should be able to bump up your Quant score a bit...
the bad news is that these classes are pretty rough. good news is that once you move on to higher maths you will learn a new way of looking at the world.