- CPU
- A Motherboard
- RAM
- A PSU
- Something to hold your OS
- Something to hold your data
- Connectivity
- A GPU
- A Case
- Cooling
- USB Ports for your ASICs
The other end of the crunching spectrum is to focus on as much cores within as small an amount of Wattage as possible -the ARM octa-cores.
In between lies the iTX approach that can also be divided in
- crazy powerfull -there are socket 2011-3 iTX boards with Xeon chipset- and
- very low power systems -a 'Pentium' N3700 has a TDP of just 6 Watt, it's brother J3710 does with .5 Watt more.
You're right, but the computational power of the iTX housed Xeon comes at a price.
Whereas the Pentium -an Atom sheep in wolf's clothing- will have more than enough power using a 60 Watt laptop brick, the Xeon is more likely to need at least a 300 Watt PSU for its 22 cores and its Quadro GPU, 600 Watt may be even likelier.
The 'Pentium' is best locked up in an as-tiny-as-possible casing, to have enough computing power per cubic feet man cave.
So my advice for Xeon iTX: as heat tends to go upward, go for a case with a horizontal placed mobo and with enough space for both intake and outake fans -and when you do not trust air for your cooling, it should also have enough room for one or more radiators. The Bitfenix Prodigy was among the first cases that took this iTX approach -still a bit limited in the PSU deepness dimension-, later followed with e.g the Cooler Master HAF Stacker 915R/F, the Cooltek W1, the Fractal Design Define Nano S, the Phantek Enthoo Evolv ITX and the Thermaltake Core X1.
For Bay Trail, Braswell or AMD AM1 my advice is to go for cases like the Antec ISK 110 or one of several Inter-Tech cases with internal 60 Watt PSU.