Check that your cabling isn't restricting airflow. Try and pin as much to the bottom of the case as possible and reroute it underneath the board as much as possible, and try and push as much as you can out of the way of the fans. Some cable ties will come in handy.
Move the fans so that one is blowing between your PSU and second GPU. This can get hot and the PSU can draw cool air away from the second GPU.
Check your case fan speeds, they're set to low as default and I found that to not be enough when running Crossfire.
The rear and top fan should be outtake by default. The front fans should be intake. You can test this by laying a piece of paper across the fan grill and see if it sticks to it or blows off it.
I also have a 120mm side fan blowing onto the GPUs.
Here's some pics:Cabling with no modification:Fan positioning:Edited by Lifeshield - 2/12/11 at 4:44am