Should I go back to school to improve my low GPA to get into an MBA program?

They'll see you as someone who flaked (and pretty badly) during college. However, if you're able to pick yourself up and get good grades and a stellar GMAT score, they'll recognize that also. A second-tier in DC is a definite stretch right now, but several years of taking classes to up your GPA will definitely help. And do lots of leadership and/or extracurriculars that are business-related. The admissions committee will want to know (and guarantee) that you won't come to their school and just get F's and W's again.