May 21 (Reuters) - *Goldman Sachs sees S&P 500 at 1,750 by year end 2013 *Goldman Sachs sees S&P 500 at 1,900 by 2014; 2,100 by 2015 *Goldman Sachs forecasts dividends will rise by 30 pct in next two years, dividend yields likely to stay around 2 pct ...