My friend and I were in South America for 4 weeks in February and believe me, that wasn't enough time! We landed in Buenos Aires, were there for a few days, took a bus to Mendoza and were there for 4 days, then back to BA for another couple days, spent 2 days in Iguazu on both sides, spent 5 days in the Amazon (missed out on Manaus because of time constraints), and 7 days in Rio during Carnival. If I had put insurance on my flights in the US, we would have stayed longer!
If we had two extra weeks, to add up to you, I would have added a couple days in Uruguay, Patagonia (I even wanted to fly to the Falklands!), Cordoba, La Salta, The Pampas. Or add a week in Chile and choose which other places you really want to see in a weeks time. Also look into airpasses. Cheaper to get while in the US before you go on you're trip and you must have your plane tickets purchased before you can get your airpasses. Buses are nice in Argentina. And during Carnaval, you must get your place early because they fill up fast! I say you make a list of what you really want to do and see if it is more important to see everything or pick out the places you must go and plan around that.
Trust me, at first I was thinking out itinerary was Argentina, Brazil, Guyana, Chile, and Falkland Island! Looking at the timeline and what we could do, we ended up just moving it down to Argentina and Brazil, which I'm glad we did, though I still feel there is more to see and I definitely plan on returning soon for a bigger adventure! Anyways, if you need anything or have any questions, let me know! I still have some information from my trip in my head! -snip-
[ Edit: Sorry, no promos please. ]