In theory, if you are holding a US passport you require a Visa to enter Brazil. However, in practice, its been reported that if you enter Foz de Iguazu (Brazil) by land from Iguazu (Argentina), you will not be required to have a Visa. Most reports of Visa free entry into Brazil in this fashion are for cases where tourists book a car from the Argentine side, and spend a day over there and then return. It seems that the local economy in Fos de Iguazy doesnt want to miss out on the day tourism.

As for the Chile entry.... Im not sure if Chile is requiring Visa to US citizens, so Im sorry I dont have any details on this. However, border crossings are very normal and I dont know of any special hassles in doing this.