Well, the thing about these cards that I find annoying
Annoying yes, but well worth it if you get them working properly. The worst issue right now is that there can't be more than a 125 Mhz delta between the core and memory clocks. Pushing the mem clock down from 810/1250 to 810/685 brought the temperature down by over 10C, so the memory is definitely a problem there. Other than that, they're good cards.
I'm using 2x6950 Asus DCUII 1Gb cards; both are set to 850/725 @ 80% fan speed using the following:
atitweak --set-engine-clock=850 --set-memory-clock=725 --set-fan-speed=80
You can also use aticonfig, I've just come to like atitweak.
aticonfig --adapter=all --odsc 850,725
The cards are currently 68C and 71C with no case and only a weak box fan that does a lousy job clearing heated air from the dead space between the cards. Dropping fan speeds down to 60% brings temperatures to 72-76C and 50% goes to 75-78C.
Currently running Catalyst 11.6/APP 2.4SDK and Phoenix 1.50 using the latest phatk kernel with options "VECTORS BFI_INT AGGRESSION=12". They're pushing ~345 MH/s with those settings. During cooler times of day, they get kicked up to 880/755 @ 80% which gives ~360 MH/s and temps of 70-74C.
I have yet to attempt flashing the BIOS or changing voltage. Even with all else being stock, I can push them to about 920 Mhz and brush up against 400 MH/s, although the temp gets into the 80s and they aren't completely stable. With voltage control and the ability to underclock mem more than 125 Mhz below core, I'm fairly confident I could approach 1 Ghz and easily exceed 400 MH/s while still keeping the temp around 85C or below.
It's tempting, but I don't want to install Windows (outside of a VM to run RBE) to make those gains. I'll be testing Hashkill soon to see if that offers any improvement. Otherwise BIOS flashing seems to be in order - and maybe some Arctic Silver. At least that can be done with a FreeDOS USB drive and a screwdriver.