Just to add some of my small experience with the 6990's using Windows, I set the clocks 920/840 and get just under 400 MH/s per core(just under 800MH/s per card) with fans set at a constant 90% the GPUs get around 70-75c except for GPU2 which is at 81c. I noticed when going lower with mem clock it actually gains heat (1-2 degree's). Also these is a switch near the crossfire port that will turn stock 830/1250 into 880/1250, I used afterburner to get the 920/840. Also something to think about is that you can not under clock memory less than 100mhz of core clocks(you can visually do it but the effectivness is zero and will reset your mem clock back 1250, at least in linux anyway). I did get over 405 MH/s by setting the clocks to 940/840 but didn't want the extra heat or didn't want to increase fan speeds above 90%. The temps are stable and barely move. I used Phoenix Riser with "vectors" "13 aggression" and use the "BFI_INT" as extra flags. I still need to tinker around with the fan speeds to see what effect it has when lowering them, but im happy with 2 x 6990's(I use to have 3 but got too hot under linux) which produces just under 1600 MH/s