Amat-
I didn't know about C&A!! Thanks for the heads up.
Have you tried Orix or Normandie? They are both really good places for sweaters, skirts, shirts, etc. They tend to carry more "work separates" (at least to me) than going out stuff, though. I´ve seen Orix scattered around the city (Santa Fe, Florida, Corrientes) and I know that there's a Normandie on Bulnes and Santa Fe and another on Florida.
My all-time favorite store here though is Tramps. If you haven't been there don't let the name and the pleather cocktail dresses in the windows fool you: they have a ton of great t-shirts, sweaters and going-out shirts. I would put them on a very similar level as Forevever 21 and HM in terms of style, price and quality. The only one I know of is on Corrientes and Pasteur, but I'm sure there are more- I just haven't seen them. Check out Once/Almagro along Corrientes. It's a pretty big shopping district with shoe stores, clothes, jewelery, home goods, electronics, etc. But, a lot of the stores only sell to mayoristas or have a minimum purchase, so be sure to ask when you go in.
Also, don't forget about the outlets stores! Blaque, Prune, Ona Saenz, Port Said, Lacoste, Wanama, etc. all have outlet stores along Cordoba. They're mostly clustered around Cordoba and Scalibrini Ortiz, but some of them are along Gurrechaga.
Hope this helps!