Definitely go to Galería Jardín (Florida & Tucuman), it's the largest concentration of computer stores in one location in Argentina.
Plan B: walk Florida in any direction and check Falabela for their discounted electronics table. They have more stores going towards Plaza de Mayo.
If you walk Florida you'll see Compumundo, Musimundo, Fravega, Garbarino etc. All walkind distance from one another.
I would go into the individual stores and ask as well about discounts. If you have a friend here (or a friend of a friend), sometimes a store will offer a particular discount for using a particular credit card 10%. Or, others will give you a break if you pay in full in cash. I just went through this with a friend buying a computer, and we tried ordering an IBM from the States around Christmas last year. It was back-ordered due to flooding and delays from one of their parts manufacturers in Asia.
We started comparing prices and features here, and eventually ended up with a local one, BGH, which he's had for months now and is happy with, the only thing is the built in camera doesn't seem to work. Anyways, your shopping experience will vary here from that in the States. There is typically only one or two models available for imported brands. The laptop will likely be at least a generation old from those in the States. I didn't see any store with solid state memory or the newest gen processor. I think everyone I saw came with Windows 7. Most stores have what appear to be the same models.
Also, I think people tend to try to keep costs down here by having a short battery life. You will likely have to run it plugged in as they typically have 2 hour battery life. Luckily, most cafes do actually have outlets if you memorize where those tables are in your favorites .
That would be my guess as my friends from Chile seem to have Mac products. I did a quick google (santiago, chile mac) and the prices seem ok here, it's just one authorized store: http://www.globalmac.cl/MacBook-Pro/
I'm not sure what currency these are shown in (maybe US dollars?) because the Chilean peso would probably show prices in the millions. There might be more stores to check out.