Hey guys thought I would just throw out my settings and software to maybe help other that are in the same boat I was in a couple of weeks ago(new people).
Operating System:Windows 7 X64
Aero Theme Disabled
Drivers:11.6 or 11.7 preview
Software:Phoenix 1.50(with 3% patch to phat kernal)
Aoclbf: Vectors - HD5xx - phatk - Worksize(128), Aggression(7)
or
GUIminer: Miner(Phoenix with 3% patch to phat kernal), Flags(+k phatk VECTORS BFI_INT AGGRESSION=7 WORKSIZE=128)
GPU Clocks:Core: 910 or 880(hash rate bouces around more)
Mem: 840
Volts: Leave OC switch on cards in default position, No need to overclock
Cooling:60% Fan speed on cards
Linda box fan @ second speed
Temps:Hottest: 72c
Coolest: 62c
Hash Rate:432-455 MH/s per GPU
Details:I currently have 3 x 6990 in one box, using a 890FX-UD7 board I was able to get good spacing between the cards, I also setup a linda box fan with cardboard wind tunnel directing the air straight onto the cards(not need but reduce the temps alot) Anyway get your self a copy of aoclbf front end, I suggest this because you can overclock/underclock your cards from the gui, and MSI after burn had faults when the 3rd card was put in.. Get your self a copy of the Phoenix 1.50 and apply the 3% mod in the kernal(open in text editor) I found for the 6990 cards the best memory clock setting is 840 anything below increases heat and is unstable and anything above just increases heat. also set your core clocks 910, so you should have all GPU's set to 910/840 while keeping the OC switch on the cards in its default position. in aoclbf tick vectors, 5HDxx+ , phatk and set your worksize to 128 and aggression 7, yes 7 because higher aggression will give you less MH/s and lower will give you less MH/s. Or if you prefer to use GUIminer or straight console the use these flags "-k phatk VECTORS BFI_INT WORKSIZE=128 AGGRESSION=7" also make sure to use drivers 11.6 or preview drivers 11.7 for each GPU you should get 432 MH/s and bounces to 455 MH/s back and forth, which brings a grand total of around 2600 MH/s to 2700 MH/s if you have 3 cards like me. having these core clocks set lower than most OC'ers and keeping the OC switch in its default posistion not only reduces cost due to using the standard voltage but also keeps your cards cooler, my 3 6990's never go above 70c(card fan speed at 60%) with the linda box fan wind tunnel(set to speed 2 out of 3). Of cause this is only a base for you to work from, you can adjust to your desire but I found this to be efficient and fast.
3 x 6990 SettupBoxfan(HeadLess)If you have any 6990 settings you would like to share, feel free to post them and im sure with every ones help we could come up with a optimal settings for these cards.