Why ask for help for a problem that doesn't exist? As was said, stock cooler will be fine especially if you turn the fans up. Make sure you have good case airflow. What psu do you have?
I would not add another gtx 285 with that psu. It's just a mediocre quality and you'd be pushing it to it's limits with another card. You should get at least a Corsair 750W, and would be better off with a 850W.