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I guess I'm too green to post where I wanted to, so I'll post here instead.

Waramp22 and others asked about success using this combination of gear.

I am in the process of building a 4 card miner a la http://www.coinminingrigs.com/quad-7970-mining-rig/ (http://www.coinminingrigs.com/quad-7970-mining-rig/).

I decided upon 7970s only because the holiday craziness seems to have bought up every 7990 on the planet, and I'm not paying $600 for a used 7950.  In any case, I have an eVGA 1300W PSU, and most of the primary or secondary components that the author describes.  If you want the complete list I can do that, just ask.

In any case, the particular mobo I bought, has 5 PCIE slots, and only one is a 1x slot.

The GPU in question is a  Sapphire 3GB GDDR5 384bit with Dual-X fans. 

Running two of them is 16x slots, I get one running at around 540MH/s, and the other at 750MH/s or so.  I start with the following parms:

cgminer --scrypt -I 13,13 -g 2 -w 256,256 --thread-concurrency 8192,8192

I further tweak both GPUs as follows:
E: 1085
M: 1500
P: 18

Intensity for these two cards is set at 13.  I can probably teak more, but was trying to get the full rig stable at any speed before I went nuts with tuning.

This gets me the above numbers and is extremely stable.  So, what about the other two cards?  Well, I haven't attempted 4 GPUs yet, as the 4th just showed up yesterday.  However, when I add a third card, I have to do it via a 1x to 16x powered riser cable, as the fan housings on the GPUs appear to take up a bit more space that you'd want them to, and you can mount the GPUs in every other 16x slot, as a result.  I haven't moved the riser to a different card just yet, but I suppose that's next.

If I just start up cgminer with all three cards, two (GPU0 and GPU1) of them take off, and the third  (GPU2) runs at 20-30MH/s.  If I change the placement of the riser cable to various open slots, GPU2 is always in this 20-30 MH/s range.  I can tweak GPU0 and GPU1, but as soon as I touch GPU2's settings, the driver fails, the card goes SICK, etc.

The Intensity of GPU2 is set at 8.  If I tweak anything else in the GUI menu, it tends to fail after a few minutes.

Any ideas on how to proceed?  I don't think I want to remove the fan housings, but perhaps that wont be too difficult.  I'm pretty sure it's not a power issue.  How does the bus figure out which card is GPU0?  Do I need to use the three resistor trick on that third GPU?