I was a Peace Corps volunteer in Ethiopia (97-99). I wasn't a nurse at the time, but it was my experience there that sent me back to school to become one. It is a perspective changing experience to say the least.
Joining the Peace corps is great if you have 2 years to spare and don't have kids. Unlike MSF, they place you in stable areas of the world (although there are no guarantees). And, most likely you will be in a town/village by yourself so you will really get a taste of life in your host country. I'm sure your skills as an RN will be greatly appreciated.
If you don't want to spend that kind of time in another country, my friend (also a fellow RPCV and now an RN like me) told me about an organization where you can spend as little as a few weeks to a year or so providing medical care in various countries. It's called Catholic Medical missions or something like that. The website is www.cmmb.org
. I think you can bring kids but I'm not sure.
Good luck in your decision making. MSF is a wonderful idea, but if it's your first time working in a developing country I would recommend first going with a smaller NGO or PC, just to get your feet wet.