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I'm suddenly interested in having 3x Radeon 6990's on one motherboard. This is 6 GPUs total.

I am aware that crossfire will only be able to address one of the cards, but I will not be using them for gaming, so I should be able to address the cards for computational purposes correct?

What motherboard would work for this? I saw that someone had benchmarked 3x Radeon 6990s for a computation but that blurb did not mention how the cards were being addressed or have details about the rig.

Also since I won't be saturating the PCI bandwidth then it wouldn't be a problem to use the 8x and 4x slots.

Finally, I'm not sure what kind of power supply I should be looking at. 1600w?

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Only two of the three cards can be connected together for CrossFireX. The remaining card will operate separately and can be accessed as a separate display adapter. If you do not, you can access each card separately. You will likely need a motherboard like the EVGA Classified SR-2, an Intel Xeon processor (possibly two), and a case sufficiently large to fit the motherboard. The total TDP of the processor(s) cannot be more than about 200 to 250 watts.

If the Unlocking Switch is not set on any of the cards, the maximum power draw from the cards is 1125 watts; if set on all cards, 1350 watts. A 1600W power supply can give enough power to run the cards without the Unlocking Switch set plus a fast CPU and other parts, but this cannot be guaranteed if the Unlocking Switch is set on one or more cards. Make sure that the total load of all parts, including the graphics cards, processor(s), and other parts, does not exceed the capacity of the power supply.

From the content of your question, I am assuming that you are using the cards for Folding@home; if configured properly, you should be able to use all three cards. Remember that the system must be placed in a well-ventilated location and properly cooled (preferably liquid cooled).

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Yes, I will be considering liquid cooling. I want to access each card separately. I prefer to only use one processor, a xeon? Can I just use something like an i7 920 or whatever fits in that sits. The CPU and Memory do not need to be that great. And I would like the machine to be silent. –  cqm Jun 16 '11 at 1:25
    
Yes, a single processor can be inserted into the motherboard. The Xeon E5620 and other E5500 and E5600 series processors can be considered for use on this system. –  DragonLord Jun 16 '11 at 1:57
    
How do I power a power supply larger than 1300W? I am in the US and 110V AC is limited to 1300W, two plugs? lol –  cqm Jun 16 '11 at 2:26
    
The limit is actually 1875W from the wall socket (about 1700W considering losses in the PSU): 125V maximum voltage, 15A. –  DragonLord Jun 16 '11 at 2:29
    
Any suggestions for a power supply? Newegg only shows 1500W –  cqm Jun 16 '11 at 2:32
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