Depending on what you want to do, I disagree with emgasu. If your plan is to do any hiking (even daywalks) in southern Patagonia do not go in the winter unless you are already experienced with hiking in the snow and ice, potentially with some serious wind (Patagonia always has serious wind blowing. I have had to cancel a day of hiking in the summer because it was too windy). Moreover, August is quite late in the winter, so the paths may be thoroughly iced over, making crampons a necessity even for relatively short and easy walks in the valleys. Hiking on glaciers is also more dangerous than in the summer, because crevasses are hidden under a layer of snow.
That said, both Calafate and Ushuaia will offer absolutely spectacular landscapes, possibly even more so in the winter. Looking at the Perito Moreno glacier from the viewing areas is impressive at any time of the year. Another excellent option is Bariloche. In both Ushuaia and Bariloche you could take advantage of the winter to go skiing. Tthe experience in the Andes is very different to the Alps and is a good way to get some great vistas. Bariloche in general is quite well equipped for a winter holiday, with all manner of organized activities (not very cheap, though).
For great vistas in the mountains, another option is Mendoza. Winter is a lot more forgiving than in Patagonia, and when I was there in the winter we had some great horseriding, and hiking through the foothills of the Andes, as well as an excursion up a bit higher for games in the snow and some views of Aconcagua. If skiing is your thing, Mendoza has some great pistes as well, although you should check whether there is snow in advance, I have heard that the last few years there has been little snow in the area (Bariloche as well, btw, although August is a good time to go). Mendoza has some great wineries, so if you like to go wine tasting, this is a great place to go.
Buenos Aires is a big city with lots and lots of things to do. However, the mandatory tourist attractions will take you about 3 days (3 full days). I really like Bs. As. and recommend spending more than 3 days there, just to take it a bit easier and not rush from one sight to another, but it depends a lot on what you want for holiday.
Iguazu takes 2 days. Not sure from Bs. As. as I have planned trips coming in from Brazil, which has a bigger airport, but with some luck you can catch a morning flight straight from Bs. As., spend two days seeing the waterfall (one day on each side: Brazilian side does not take as much time, but has a great bird park right there too), and then fly back to Bs. As. in the evening. I know for a fact that this plan works from Rio or São Paulo, so hopefully it does from Bs. As. too.
