Stocks posted nice gains Thursday, but lower volume suggested the big money remained on the sidelines. The NYSE composite advanced 2.8%, the S&P 500 2.3% and the Dow 2%. The Nasdaq trailed with a 1.8% pop. All the major indexes retook their 50-day moving averages. Volume was down across the board. An unexpectedly strong GDP number helped the stock market open on the upside. Economic growth for the third quarter came in at 3.5% vs. analysts'