I have 12 PowerColor 6870x2 and I wanted to build "that dense rig" with 4x card (hence, 8 GPU on board) multiple times. But - No luck on following issues:
1) Underclocking memory: I didn't even manage to get down from 1050. aticonfig --odsc=920,820 doesn't work. This is what all of the GPUs show afterwards:
Adapter 1 - AMD Radeon HD 6800 Series
Core (MHz) Memory (MHz)
Current Clocks : 920 1050
Current Peak : 920 820
Configurable Peak Range : [775-1000] [1050-1250]
GPU load : 99%
2) 4 Cards simultaneously: Booting with 4 cards is no problem (ST1500W SilverStone PSU installed), and they all show up in aticonfig (Linux distro). But as soon as X is starting, it bails out with a Segmentation fault. If I pull out one card and install a 5850, or a 6970, I've got 7 happily mining GPU's. The 8th one fails down the whole system. I don't believe it's a power issue or a hardware issue at all, as all of the barrier values aren't reached at all.
3) Powering clocks on anything higher than 920: Currently, thats the maximum clock frequency the cards stay stable on. Raising it to 930 kills the cards at last; they freeze at a random time afterwards. In the end, the cards stay below 300 MH/s with these settings (at 294).
I bought these cards on the better price ratio opposed to the 6990, and because of the fact almost no supplier can deliver 6990s at the moment, at least in germany. But at the moment I tend back on waiting for the (more expensive) 6990 cards, for the sake of getting at least 2,5 GH/s out of one box.
If anyone have better experience and conclusions on this device, I'd be happy to hear and share some wisdom about it.
these cards will only allow you to lower memory by 100 less than the gpu clock speed.
since you are on linux, you can flash the bios and should be able to run fine with lower memory speeds. if you flash the bios and try to run under windows, the drivers will crash.