Try it and see. I suggest keeping them below 90c at the very most, that might be hard even in a very cold room. I shoot for 75C on my miners, which can be hard to do.

For a short time (1-2 weeks) it may be OK, the cards are rated to 95c. The fans do not do well at temperatures like that, and will be the first thing to fail. Unfortunately, you'll just have to make a call. Is it worth the extra wear and tear on 2 cards for an extra 700kh/s for two weeks?

The cards should be OK running in a really cold room. I would try and stay above freezing, though. If it is a small room it will not take much time at all for it to be really, really hot! Prepare to be amazed!

No, pci-e is backwards compatible. You won't hurt a 3.0 card putting it in a 2.0 slot.

You can run three cards on a motherboard, probably, but I don't think you would want to mine with it. Two won't have any room for cooling, so they will get exceptionally hot. It takes exceptionally loud external fans to cool 3 GPUs stacked on each other like that.

I would instead run two, so that the top card has some room to breathe.

Normally, to find power supply size you add TDP of all components and add 20%. Google "TDP 7950" will get you the answer.

Anyway, it is 200W. You need 1000W for 5 GPU alone then, plus another 100-150W for the motherboard and processor. If you overclock you should plan for 225-250W per card even.

So, you need at least a 1100W power supply, but you should buy a 1200-1300W one for better results. Cheap ones are over-rated. Combining them is OK, but be mindful that you load them evenly.

here you can calculate profitability and costs http://www.coinwarz.com/cryptocurrency

start with getting ONE card working, then start adding more cards

you might be pushing it with a 750 watt psu for 5 cards

https://bitcointalk.org/index.php?board=133.0

Forumu okumamissin. 

Are u sure to be able to build a PC yourself?

As a riser cable you should use 1x to 16x PCIe extenders/risers... as I understand they should work for all slots on your MB. But if you have the possibility you should buy the extenders for your specific ports. In your case 3 pcs. of 16x to 16x and the rest 1x to 16x (2 pcs. in your case).

I'm also building one  
Here are my specs:
Gigabyte Z87X-OC with Intel® PentiumTM G3220 and Kingston 8GB 1600MHz DDR3 Non-ECC CL9 DIMM (2x4) as a base.
As hashing units I am expecting 4 MSI AMD Radeon HD 7950, 3072MB, GDDR5, 384bit to be delivered as we speak.
All this will be powered by one Sirtec - High Power PLUS GOLD 1200W - arrives on Monday... I hope.

I actually opted for this MB because I have no way to easily procure riser cables... from China they will arrive somewhere from 9 all the way to 29 of Januarry  

Have phun  

I would recommend for asrock h61 pro btc, which is claimed to be specifically designed for multi-gpu applications.

those are pci slots, not pci-e