There's a thread about ATMs in Argentina on the Argentine forum.

In Buenos Aires most places will take cards, but shall offer a discount (usually 10%) if you pay cash. In the provinces, except for touristic places (El Calafate, Bariloche, Iguazú), cash is always king. And by cash I mean Argentine pesos - US dollars are either not accepted or they will exchange them at a pretty bad rate. In my experience, euros are quite useless in South America.

Bear in mind hotels in Argentina won't charge you the 21% VAT if you are a foreign resident paying with a foreign credit card. I think the same applies in Chile (I can't remember whether it was zero VAT or a reduced rate there), and in Uruguay if you travel before April (they only have it in summer months there).