Had this kind of build in the back of my mind for long time, finally got a suitable motherboard (Tyan S7002) locally at whopping 15 euros but obviously with a quirk: CPU0 socket had several bent pins and was not working. Got them straightened up with microscope and it POSTed up nicey with two X5550 CPU's I had around. After I got the board working I ordered two X5690's: 6 cores, 12 threads each, 130W TDP 😀

Board has one 16x PCIe and three 8x PCIe slots, no onboard sound so I grabbed some SoundBlaster card with PCIe interface.
Bad thing is that the 8x slots are quite close to 16x slot which means that SoundBlaster blocks off graphics cards fan. Since there is unpopulated PCIe 8x slot too I will sloder slot there and move sound card further that way.
Graphics is suitable monster from that period: MSI GTX 480 Lightning. Maybe I will replace it with something more modern, we'll see. At this time prices for anything more modern like GTX 1060/1070 are insane. I bought GTX 1060 6GB for 120 euros two years ago and thought I paid a bit much, now they are like 300 euros, ykes (n)
So, spec list:

*Tyan S7002 dual 1366
*2x Xeon X5550 CPU's (soon X5690)
*24 GB DDR3 ECC (6* 4GB)
*MSI GTX480 Lightning
*Chieftec fulltower case
*Chieftec 750W PSU

Pics and updates coming soon.. stay tuned!