You should space each card with some foam or random stuff you find :O
If you are using a case, take the side panel off and get a fan (those standing square ones are best) and blow straight at the cards. Lowered the temps on my 6990 by 15C.
If you are going caseless, put 120mm fans on top of the cards, and other standing fans around it. Go look at the mining rig pic thread for more reference.
Make sure the room the rigs are in has good airflow. I put mine next to the window.
66.5 dBa?! You have any idea how much noise is it going to generate???
Atleast i do, Yes, 66.5 dBa
In all seriousness tho:http://www.gcaudio.com/resources/howtos/loudness.html
That is about as much as normal conversation.
Currently at my workstation the noise level is probably plenty above 66.5.
A 6990 at max load is like 70-80 decibels. :O