Just to confirm other posts. Get a Nationwide Flex account - it's saved us loads of money in fees on our year away.
November 07 in Argentina ATMs only lets you draw 300 pesos out at a time - around 47 english pounds - so you have to make several withdrawals at a time which would incur a service fee each time. And yes carry a few hundred USD with you for back up - but in some S. American countries you can draw these out of ATMs. We had no problems with ATMs though anywhere in S.America - and yes there is one in Uyuni that accepts visa cards.