Good HPC motherboard to support dual Nvidia C2075 running Ubuntu 12.04

Not sure if this is the correct subforum to post this question - I didn’t see a Hardware forum.
I’m trying to find parts/motherboard for the following system:

-Supports 2 Nvidia Tesla C2075 cards
-Ideally has usable 3 leftover PCIe slots (one 8x, two 4x) I could live with 2 leftover slots 
-I'm putting it into a 3U rackmount box with 15" or more of depth.
-Needs to run Ubuntu 12.04 x86_64

-What kind (DDR?) of RAM should I use with these cards and generally how much do you need to generally support 2 C2075 cards?  I know, it depends on my application, but what's the min?
-How big of a power supply will I need?

What motherboards are people using for this type of HPC workstation? What is your experience running CUDA applications with Ubuntu 12.04 and dual Tesla C2075?