I spend 4 1/2 months in South America and I would recommend that you don't want to see to many countries(there is plenty to see in south amerika).
I just did Peru, Bolivia, Chile en Argentina. I spend about a month a country and still you cant see everything you want to see.
The Inca Trail to Machu Pichu is really wonderful (February is the only month you can't do the inca trail, rainy season). I thought that there where to many tourist in Machu Pichu but it is very nice. Cusco is also a nice place to party and to meet people ( I also did Arequipa, Puno, Nasca, Pisco en Ica).
In Bolivia there are a lot of things to visit and do around La Paz: Climb a 6088m high mountain, Mountain bike the Death road, various treks and Titikaka lake. Sucre is a nice city, In Potosi you can visit the mines, Uyuni is a must see. In Bolivia there many other very nice places to visit (Rurenbaque, Copacabanac, Cochabamba, Santacruz and many others).
In The north of Chile and Argentina you visit Salta and/or the San pedro, then I would take the plane to the south and visit patagonia: El Calafate, Ushuaia, Torres Del Paine.
That is pretty much what i did. But you could also see only the highlights of more countries. I is for you to decide.
We didn't travel slow, just a couple a days in each place but there is plenty to see in those countries.
Have fun.