Here is a Ted talk that prevents data about inequality: http://www.ted.com/talks/richard_wilkinson.html

There's another take on it here: http://www.quora.com/Economic-Inequality/How-is-income-inequ.... In, that link says that the degree of inequality matters in the us because in the us, the poor are desperately poor, and the rich are fantastically rich. In other words, if the poor had a pretty good standard of living and the rich were absurdly healthy, maybe that would be ok, but that's not the situation we have.