You want to be really careful on timing this one. According to both the official and "blue" dollar rates the peso is devaluating rapidly. The official rate has hit 8 pesos to the dollar today, according to La Nación (article in Spanish):

http://www.lanacion.com.ar/1657796-c...icial-blue-hoy

With the peso devaluating more than 5% per day at present I think you're best to bring cash to BsAs and change there.

An interesting alternative for changing large amounts would be to go to Punta del Este (in Uruguay) where the exchange rate is reported to be even more favorable dollar-wise, then cross over to Argentina. But I don't know how the authorities look on foreigners importing large amounts of Argentine currency.

I have to ask though, given the instability of the peso, why would anyone want to change large amounts of dollars into pesos?