I built a dual e5-2670 sr0kx system out of z820 parts I acquired on ebay.
Hindsight being 20/20 and now knowing about natex I wouldn't have gone this route, but after reading the techspot article I was on the hunt for a motherboard on Ebay. Without thinking, or should I saw "knowing" - that HP was just proprietary bs, I bought the motherboard NIB, reading some more about my purchase it dawned on me I'd be endlessly hacking a power supply to make it work, but then I found a cheapish (they accepted my offer) z820 case w/ 850watt powersupply also sold on ebay, so it was becoming more realistic, at least without hacking things to work. The e5-2670's I got from a chinese seller on ebay, they were legit and work no problem - shipped in toilet paper, wrapped in bubble wrap though!
the z820 usually comes with a dual rad setup, which I replaced with noctua heatsinks and fans, the ram is 64gb 1866 samsung ddr3 ecc reg ram, but runs at 1600 obviously. Because i'm not using the large plastic cpu cooling shroud, with memory fans etc - I do get some errors on startup - basically it detects that those items aren't plugged in, which it thinks is critical. The shroud (which did come with the case) obviously won't fit over the noctua heatsinks, but there would be an easy way to hack that in so it thinks it's all legit. the fan connectors are different on hps proprietary bs but you just have to file down the one middle ridge on the connector.
Otherwise it works great, I haven't found a good location for the ssd yet, so it's "sitting" in the empty bay location, while a western digital black 1tb drive is through the SAS controller in the hdd bays. I don't know if HP makes a little ssd adapter for those bays, but the 2.5" to 3.5" adapter bracket I have the ssd doesn't align it right to slide into the preinstalled sata/pwr connection at the back of each bay.... the gfx card is just an old nvidia 250 gts, works perfect for straight up rendering jobs in 3ds max/vray, which is all this rig is for. It cinebench's at exactly 2000. one processor does run 10 degree's hotter than the other, but right now in vray one is at 45 - 55 range the other 55 - 64 deg range, so not bad.
I now have a Natex board on order, also with 64gb, that I will build into a phantek enthoo pro case, with dual hyper 212 evo's.
if anyone has any questions about this setup, I'll do my best!
cheers, and thanks for all the info in this forum (even though I found it too late!)