Benefits of a Post-Bacc Program.

Bad news: you're almost definitely not gonna get into the MBA programs at Columbia or Northwestern without at least 2-3 years of full-time work experience, post bacc enrollment aside.

Good news: you can probably forget about the post bacc entirely (and save a ton of money and time in the process), particularly if the only reason is to be in the running for a top MBA. From personal experience, the main use I've seen for a post bacc is in the pre-med area, where specific coursework is required. B-school admissions are a very, very different story.

There are two things you need to focus on right now if you haven't already: getting a selective, professional job that will challenge you (I am purposely vague here--this could be anything from Peace Corps, to working at a start-up, non-profit or government agency, to going to more "traditional" pre-MBA route: finance or consulting) and kicking butt on the GMAT.

If you get a very strong quant score on the GMAT, you may be able to forgo additional coursework altogether. A 3.0 will be on the lower end for top schools, but isn't so low as to require an additional degree. Plenty of people who majored in liberal arts subjects get into business school (myself included).

Good luck!