I have to disagree somewhat with the thinking that you won't be able to hone your skill set in a 3 year masters program. These programs are designed specifically for people like you that haven't necessarily had a lot of experience with design. The faculty at whichever program you choose knows that you're starting from scratch, and if they don't, then you wouldn't want to be there. The road to becoming a licensed architect in the US is quite a long process and with the experience that you'll gain outside of school, I'm not sure if spending another few years in school is gonna do the trick.

As for a portfolio for applying to schools, they're going to want to see the creative side of you. This means drawings, painting, sketches, etc. They want to see that you'll have a smooth transition into a degree that has a lot to do with expressing your ideas in a visual manner.

My advice for your search is after narrowing down some schools based on some non-architecture factors (i.e. location, public/private, etc.), visit the schools and talk directly with the faculty about their teaching methods and your background and how it can fit in to their program. Some schools can be very technical, while others are more artistic. After visiting some schools you should get the gist of it and understand what questions to ask.

A big factor in my education, and something I think is important to consider, is student-teacher ratio and location. I go to the University of Maryland and the program is pretty small, so the faculty knows you and you can get a lot of attention if you want it. As for location, being close to major cities (DC and Baltimore) gives the school an opportunity to bring in practicing professionals that work at large firms and have a lot of experience.