I just got back from Argentina a month ago. On my trip, we went to Buenos Aires, Iguazu, and Colonia, Uruguay. If you're looking to go to Iguazu and Ushuaia/El Calafate, then two days should be plenty up north. You really only need one full day to enjoy the Argentine side of the falls (we did most of the circuit trails and the boat ride through the falls). Getting to the Brazilian side, while certainly beautiful, might be a hassle for your trip. I got a visa and got it stamped, etc, even though I was there for a day, but the way I see it it's just a down payment for a future trip during the next 5 years to Rio. :-)

Don't forget to enjoy Buenos Aires. I really just enjoyed walking through all the various barrios and enjoying all the good (and cheap) restaurants throughout. The neatest thing we went to was a neighborhood milonga: it gave an interesting glimpse of the porteno culture.

Also, beware, not many people speak English!