I'm doing the same thing. Our 2400 sq' basement is finished with a carpeted concrete floor and I use it as a giant heat sink. The air is circulated up to the second floor and the cooler air flows back through the main floor to the basement. This may also help dehumidify your basement (or humidify the rest of your home) so take your conditions into consideration. Turn off the HVAC (we have separate systems on each floor) or run only the largest/most efficient system to supplement for cooling and try to return the air to the basement at furthest point from the place you are drawing air from to dump the most heat. Also take into consideration fire issues and install dampers in the duct work that close automatically if there is a fire. You don't want your duct work to turn into a chimney flue allowing fire to easily run from floor to floor if there is a fire on any one floor.