I have two 5830's and a 5770 running at around 55C each, dedicated on air.
Make sure you have as much "in force" as "out force" in the case, sometimes people place a bunch of fans inward... with no exhaust, or they place fans too close to eachother with opposing air currents.
Remember that heat rises.. so if you have cards that vent out the rear, hang the machine at a 90deg angle with some bungee cords, or a wire rack. Laying it on the side may help as well. Sometimes keeping the machine out of a case helps, but alot of heat dispesal requires alot of air actually passing though
the fins of the heatsink, not just above them.
Ambient temps factor in as well, gotta have a cool room.
Though you may just be out of luck with two 6990's, 4 GPU's in a cramped space are going to make tons of heat and there isn't much outside of watercooling that can help.