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/.
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?