Firstly, the bus journey from Santiago to Mendoza, Argentina, is BEAUTIFUL!! (So don't fall asleep, and do it in the daytime so you can see it) I loved Mendoza, and if you like red wine, this is the region where about 80% of Argentinian wine is made - lots of vineyards to explore
From Mendoza, you could head north via Cordoba, up to Salta. Salta is gorgeous, and being so close to Bolivia, it is really traditional (food and culture) compared to anywhere else in Argentina. You can go from there by bus to Iguazu (I'd recommend doing both sides of the falls - you can do both in a couple of days, basing yourself on the Argentinian side. You need a full day for that side, but taking the local bus across the border to Brazil (around $1) you can do the Brazilian side in a few hours and come back. The Brazilian side gives a panoramic view, while the Argentinian gets you in, round, down, under the falls - amazing! (If you can. I'd recommend doing the Brazilian side first - I did it the opposite way, and although still good, I think it would be better seeing the panoramic view first.
From there, you can head south to Buenos Aires (perhaps going via Uruguay if you have time, as you can go there by bus, then take a boat to BA)
I didn't do Patagonia in Argentina and loads of people rave about it - if I went back, I'd try and cover that, so it may be worth looking into too - though the route I've suggested above would be good in a month, so I'm not sure how you'd fit Patagonia in to that, since it's south Argentina.
Let me know if you have any more Q's. My 'big trip' blog below contains entries from each of the places I've mentioned if you want to see photo's / view what I experienced, but also as Corinne said, you can do searches via the website too which is useful. Happy to answer more Q's though - I could talk about South America all day long if you let me