The MSI Big Bang, for about $300, has six (six) PCIe slots on it.
Can this baby pump the juice? I'm talking a 1500W power supply, and six video cards hooked up with PCIe extension cables and a channeled box fan.
I'm just worried that the motherboard will get fried by 1500 watts. If the PSU can handle it and there are enough 6 pin connectors I should be okay, right?
What about using two power supplies? I've heard that's possible too.http://www.gigabyte.com/products/product-page.aspx?pid=3880#ov
There's another 6 slot board, with space for four cards without extender cables.
I've heard the gpu limit is 4 in Windows, but I've also heard people have had more. I know in Linux there is no actual limit, I believe there is a simple kernel setting to achieve this.
But most of the 5+ gpu setups I've seen where dual gpu cards. I've read a lot of reports that using 5 cards causes problems with a lot of motherboards. In theory 7 cards would work with Linux but I wouldn't have much confidence the motherboard is going to support it. I'm not sure what the main problem is with 5+ cards but I've seen enough posts about it that I'm fairly sure it's legit. There was a guy who had 4x6990 8 gpu linux rig and he said adding the 5th card (6990 and a non 6990) he couldn't get it to work.
You're better off with a 3-4 pcie system. The mobo is cheaper, the psu would be cheaper and from my numbers the 5-6-7 card systems don't give you more bang for your buck due to the mobo/psu costing quite a bit more.