TimB,
Peace Corps isn't a particularly competitive program. It is, however, time consuming. Think about a minimum of 6-9 months from application to arriving overseas (for many it is closer to 12), and then a 27 month commitment. I doubt having a graduate degree would make that process go faster, and I'm not sure it would make an impact on the type of assignment you would get (if you are applying with your wife, PC will have to find two jobs in the same location you are both qualified for). Also, I believe Peace Corps requires couples to be married for at least a year before they begin their service together.
If you feel like you can wait three years before starting grad school, I would recommend Peace Corps because having that international experience will make you that much more desirable as a candidate for graduate programs. However, as hedong123 said, if you already have lived and worked overseas, you could probably apply to grad school now.
As for your quant score, I think you're falling just short of the middle 50% for the better programs, so I don't think you need to take it again. However, math is important (especially if you want to study international development) and taking a night course or two in calculus or economics would be great (and maybe necessary) for your application.