The goal was to build a small, near silent, powerful machine learning system using known high quality parts. I chose the Fractal Design Define Mini C MicroATX Mid Tower Case for its size and quietness. An Intel 8700K was preferred over an AMD Threadripper for its power/performance trade-off. The ASROCK Z370M Pro4 motherboard has two 16x PCI 3.0 slots, possibility for M.2 Raid, and four DDR4 slots supporting 64GB but no WiFi. The Seasonic power supply was a fit for the case, quiet, on sale, albeit with 850W is over-powered for the build. The MSI GTX 1080 Ti Gaming X Trio is a top of the line GPU for machine learning needs and is particularly quiet. Despite a warning from PcPartPicker, the GPU really fits once the fan is put in the alternate location (see photos). I got 2x16GB DDR4-3200 to leave room for an additional 32GB, if needed.
The newly available Intel Optane SSD 900P 480GB is reviewed to outperform other solutions such as NVMe M.2 cards. I got the PCIe version but might have opted for the U.2 version thus freeing up a PCIe slot. Unfortunately a 480GB U.2 was not yet released.
Update 18-Nov-2017: The idle cores are around 45C. Under Prime95, overclocking at 4300MHz to 4500MHz, cores show 80C. I'm mostly interested in overclocking up to the point that the fans increase in loudness which is a very modest overclock.
** Using either Prime95 or Intel Extreme Tuning Utility Stress Test benchmarks.
Update 19-Nov-2017: How quiet can it be? I removed side panels and disabled the two chassis fans. Overclocked temps soared to above 90C while the CPU fan roared. Next, I tried lowering the CPU maximum clock until the rig became near silent. At maximum CPU clock of 3900MHz, I can't hear any fan noise. Running Prime95 on all CPUs, temps stayed between 48C and 68C. Idle temps were 26C to 32C. This is a very good baseline for silence. I'll experiment with increasing performance while keeping it as quiet.
Update 23-Nov-2017: Replaced stock Fractal fans with Artic Silent. They are indeed quieter than stock Fractal. Front 120mm changed to 140mm correcting direction. With max CPU clock of 3900MHz temps dropped 5C to 48C to 63C. With max CPU clock of 4200MHz temps were 55C to 70C. With max CPU clock of 4400MHz temps were 58C to 76C (quiet but not as silent). With max CPU clock of 4600MHz temps were 60C to 80C (borderline quiet, definitely quieter than my notebook fans). With max CPU clock of 4800MHz temps were 60C to 83C (notebook fan loud). With max CPU clock of 5000MHz -- crash.
Update 7-Dec-2017: Running day-to-day with ASROCK A-Tuning software settings at 48x CPU, 46x CPU cache, -.05 Vcore voltage (offset). Temps are 31C idle, 78C max (100% CPU and 3DMark Extreme stress tests). Temps are very dependent on A-Tuning FAN-Tastic settings. I optimize RPMs for silence instead of cooling/performance.
Update 23-Dec-2017: I've returned to running the CPU clocks with modest overclocking (CPU 4400MHz (nominal 3700MHz), CPU cache 3700MHz). The system runs 24x7. The system has awesome power which easily handles day-to-day tasks (idle is 26C, average is 30C-36C, max is 65C-80C). So far, the machine learning tasks are GPU bound so little need for overclocking. I know 4800MHz is available whenever I need it by simply by starting up ASRock's A-Tuning software. Ambient sound level in the room is 44db. The system adds no measurable noise unless really stressed in which case, measuring at the fans, 47db, 54db and sometimes as high as 64db (overclocked to 4800MHz,100% all cores plus 100% GPU).