It is slightly easier to see where I hope to go with this image:
Overall structure is formed with thesis, expansion, and recapitulation as one might expect. I think Paul is addressing the question that Milton reflected on in Paradise Lost - "to justify the ways of God to Man".
His questions are specific: Framing the argument: what is the advantage of those under the covenant? Within the argument: how to we get out of trouble? is the law good? is God righteous? And his exploration of pairs of words in the central section, Sin and grace; law and grace; law and sin; Spirit and flesh; creation and hope, reflects much of our future dialogue (hah - no kidding).
Here's a better overall thumbnail of the letter.