I actually don't think that you need an MBA for those things. An MBA definitely doesn't hurt, but what you're talking about sounds fairly specialized. If you're not sure about an MBA, I think some schools offer other graduate programs in Organizational Behavior and related topics. You can get a Masters or even a PhD in those fields.
As for the GMAT, it's definitely not THAT hard! Our advice is just to give yourself a lot of time to prepare. Hopefully you've already started. Buy a book or two, including
the Official Guide for GMAT Review. If you still feel unprepared after reading those, then consider taking a class such as Kaplan or Veritas. But, with enough time to prepare you should be fine.
Scott