Hey Nick....I've been planning to travel quite a bit in SA in 1-2 yrs, and my thought for a while has been to study for 4-5 months in Buenos Aires since it seems to be a pretty badass city. From what I've heard there are great language schools throughout the continent though, so really you could do it anywhere.
As for accreditation, the Universidad de Buenos Aires offers a language program for foreigners called the GIC. Here's their website: http://www.gicarg.org/?tabid=85
The university itself seems to be large and well respected, so the course work is legit college credit that should be transferable: http://en.wikipedia.org/wiki/University_of_Buenos_Aires
I think it is a little more expensive than the informal schools, but not by a whole lot...maybe 20-50%? Not peanuts but I guess it all depends on your budget. It does include some extras like access to a health club and use of all the university computer labs and weekends trips, etc. I was trying to decide between GIC and Ibero Spanish which is an informal school that looked pretty cool. I always like the idea of real credit, but I figure the unofficial schools probably attract more of a backpacker crowd. Either way it probably works out well...
If you do this I need reviews on whatever you choose to do! :D