tom's case cooling guides:http://www.tomshardware.com/reviews/cooling-airflow-heatsink,3053.htmlhttp://www.tomshardware.com/reviews/cooling-air-pressure-heatsink,3058.html
that case should accommodate 2 560s you should be fine as long as you remember to keep a good gap in between gpus. coming from my hafx and v6gt, either horizontal or vertical is fine, and wont matter much, although you'd want to have incidental airflow coming from the heatsink fan hitting the VRM sinks on the board, if any.
EDIT: side panel fan didn't help much with my crossfire temps many moons ago, but an additional top mounted fan exhaust should help with convection when things get hot. reference video cards will help keeping the case temperatures too because they exhaust hot air directly out of the case instead of dumping most of it inside especially if you have 2 of them running.