My rig has 4x GPU's in a milk crate (janky, but effective!). Their close proximity was causing some cooling issues to I had my girlfriend laser cut a custom "lid" out of acrylic for it that has 4X fans ( http://www.amazon.com/Cooler-Master-Bearing-Computer-Radiators/dp/B000O8I474/ref=sr_1_2?ie=UTF8&qid=1392332086&sr=8-2&keywords=coolermaster+fans ) on the top of it. This brought temps down ~5 degrees, GPU's are now sitting pretty at ~70-74 C.

So all in all, it depends on how you're arranging your rig. Some of the larger, more open rigs have plenty of space to cool, but especially if you go with the ASUS as you have said in your later posts, they direct exhaust air to the side towards the next GPU in line, so the first one in the line gets plenty of cool air, and the ones to the side get heated air from the one to the left. Adding fans to push this hot air away keeps the system much cooler.

Best of luck!