Well to me this is really a trade for Brees and I don't think that you really need to make this move. There is talk that Warner is set to retire this off season and it will leave you with some solid options at qb with Palmer, Leinart and Sanchez. I'm not sold on Forte, mainly because there offensive line is a complete mess, they can't run block and Chicago has not early draft picks because of the Cutler trade, so this may not change for another few years. I would stick with what you have for now.
The Palmer to Brees upgrade is huge, but I think you are giving way too much. We know Calvin can be big if he can just stay healthy. And it's scary to think what Charles might be able to do. I'd sit pat.