Club América is a popular Mexican soccer team (and also really hated) which has crazy fans. Most girls in Mexico named América are named by their fathers in honor of the soccer team. I don't blame them because if "Chivas" were a name I would also name my daughter that in honor of the best Mexican soccer team in the world!
In Brazilian Portuguese the emphasis is different: ah-MEH-ree-kah. I like this better, but the Spanish is still nicer than the English prn.
It's a lovely name, but maybe not so much for English speakers. Which is sadly why I would never use personally. America Ferrera carries it beautifully, though, in any pronunciation :)