The company that designed the custom case I used I believe used ASRock motherboards as well as ASUS. I personally had very bad experience with the ASRock Supercomputer motherboard with RedHat Enterprise Linux, whereas the ASUS Supercomputer motherboard worked wonderfully. The price different is a few hundred dollars, but if you are already spending that much on four GTX 295 cards, I think it is worth it.
And yes, I think for this many GPUs, the Core i7 series with triple channel memory is the way to go.
Yeah, that also looks fine. I wanted something small that could slide onto an existing shelf, and I wanted to test out Protocase as a company for potential future jobs. That custom case was $300 (much less if you want several) and is about as small as you can build a quad-card system without going to liquid cooling. The original designer wanted a computer that would fit in carry-on luggage. My space constraints were not quite so extreme. :)
You can also find much cheaper cases by Antec and Li-Li which have 8-10 slots in the back (no PSU), and therefore could also hold four GTX 295 cards.