I think the only person that can give you any real answers is Tom, I don't think anyone else here has one or knows anything other than what they have read or seen in reviews.
My case is the Raven RV02. I have been worrying about the case size since i ordered this card. My case can fit a 5970 EXACTLY with the 180mm fan grill touching the 5970. and the 5970 is 11.5 inches while the 6990 is just about 2 or 3 mm under 12 inches, so i may have to remove the fan grill. i hope it does not come to the point to were i have to remove the whole fan itself. and even if it doesn't fit i will put it on my test bench until i find a bigger case that offerers the same amazing airflow my Raven does.
But sound i am not worried about. my case has high density sound proofing foam all over the inside of the case, special order from klipsch
about 7 months ago i had dual reference 5770s (xfire) and at 100% fan speed it was whisper quiet. all i could hear was the air escaping my case and a faint fan in the background that was pretty soothing. You just have to find the right place to get the BEST foam. my foam is some of the best studio foam you can buy (dont mean to brag). cost me just under $200 just for the foam. a little expensive but WAY worth it. besides i plan of watercooling.