The main indexes headed to a strong close Wednesday as volume spiked in afternoon trading. The Nasdaq was up 0.8% and above the 5000 level. The S&P 500 climbed 0.7%, while the Dow Jones industrial average rose 0.6%. Volume was tracking more than 12% above Tuesday’s pace on the Nasdaq and more than 20% on the NYSE. Breadth also got better as the session progressed, with winners topping losers by nearly 5 to 2 across the board.