I have a fractal design r4 with stock cooling fans. I have a matx mobo (asus gryphon) and have just brought a 280x to crossfire with it. The fans on the gpus and cpus sound like a jet engine and reach 90 degrees when I crossfire the cards.
Note there is no space between the cards in my case, they are literally sandwiched together due to the matx mobo. I was thinking about using the extra case fan slots to add more cooling. Would this help or I am screwed and need to sell my card and live with a one card system.
You can try and find a 3rd party cooler for your 7970, im sure there arent any out specifically out for the R9 280x yet. I have an Arctic Chill 7970 edition or something along those lines. Check newegg for VGA cooling solutions, or build a custom water loop.