The covered math is at high school level, although I didn't study standard deviation when I was in high school. But I was familiar with SD concept when I studied my CFA some time ago. So no worries about basic math here.
Basic concepts are covered in the offical
OG, but they are abstract and may be hard to understand. Any other prep books will include all the math concepts, but I found "Kaplan 2007 Premier" covers it extensively and explained it very well with examples. The list of all formulas at the appendix of the Premiere is the best reference you can find. From there I completely cracked through the probability questions. After reading it carefully, probability is just about understanding what the question is asking to me.
Princeton Review's math coverage is a little simple to me. They are good reviews, but not very good if you wanna crack difficult questions.
So I hope this helps. Also why get familiar with concepts first before digging into the questions? They are the foundations the questions are built on. If you are strong at it, you can handle any math questions thrown at you. Once u are good at the basics, use many questions to practice.
Update: got an interview from the school. and waiting for the result. yes, luckily I didn't re-take the test. So if it's enough for ur range of schools, no matter Harvard or Wharton, spend ur effort elsewhere.