Those cards are going to be too close and burn up.
My 580's in SLI idle at 28C for bottom card and 32 for top card. Gaming in BF3 under load makes it 47C and 55C at close to 60 at times.
God's sake, put some cooling into that case. Heat rises from bottom to top.
I agree with the other poster get some big space between those two cards and add a BIG side fan like a Antec Big Boy or something. And stick a Server class Delta Fan or Ultra Kaze into the Back of the case and exhaust that heat out. You do have your CPU fan pointed towards the back of the case and more fans pulling, pushing out the top along with moving air across your hard drives don't you?
Please tell me that your power supply is on the bottom with a hole in the floor to fan blow the heat out the back and pull in cold from the bottom of your case.