This may be a bit too late for OP, but don't use an ATM card or credit card in Argentina if you can avoid it. Argentina has two exchange rates, an official one and an unofficial one (blue dollar rate). The blue dollar rate is approximately DOUBLE the official one (current pesos/dollar is 9.8 blue rate vs. 5.5 official rate), so it's worth the effort to figure out how to get money exchanged at the blue rate.
If you use your credit card or ATM, you get the official rate. If you exchange cash at a bank, you get the official rate. If, however, you buy Argentinian pesos in Uruguay, you can actually do better than the blue rate. You can also look into unofficial money changers, but that's kinda shady. I found that the best way to do this was to ask around at travel agencies - most will change money at something close to the blue rate, but all the unofficial routes will require that you have US paper currency on hand - and if you already started your trip (as the OP probably has), you're out the luck.
However, there is one final option, which is actually probably the best. That is to use Xoom. It's like a Western Union with pickup locations in Buenos Aires, except you can pay with your credit card in US dollars and pickup in Argentinian Pesos, at an exchange rate that usually matches the blue dollar rate.