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I've been wondering about setting up a headless machine with a lot of CPU grunt for rendering fractals. Rendering controlled from another machine on the LAN. Render jobs would be sent to it and queued and it would just quietly work away (without catching fire) in a corner of the house somewhere.

Can you still get motherboards that accept more than one processor or is that a long dead idea since multi-cores?

My desktop PC has i7 4790k processor - 4 cores - 3.6+ghz.

Then I had the realization at bedtime last night I already have another processor.... a GPU with 1024 cores...  until I discovered I might as well forget about arbitrary math on them - from a quick search during lunch break at work today.

Does anyone here use one or more headless machine(s) for this?

Just dreaming really don't have the spare cash.

Multi-socket mainboards have moved up the food chain and are generally expensive. Additionally, Intel asks for a significant premium when you want processors that work in such mainboards (you need to buy Xeon brand instead of Core-i).

It is not clear if AMD's new Ryzen processor (expected to be released in March 2017) will change that - nothing has been said about its multi-socket capability. However, the arrival of Ryzen will almost guarantee that 6-core and 8-core processors will become more affordable, and eventually the new mainstream standard (which has been hovering at four cores for a while now).

(AMD has affordable older CPU models for dual- or quad-socket machines, but I can't really recommend those.)

More than one GPU can run in parallel in the same mainboard. However, special mainboards are required if all of those GPUs need high bandwidth to the rest of the computer. Power consumption will also be pretty high when all these massively parallel GPUs are running full steam - your remark about catching fire would become relevant here. :)

Multi-GPU setups have become slightly less exotic in recent years, because they can be marketed as insanely overdone gaming rig. But for fractal purposes, usefulness of GPUs is hit or miss, depending on the software being able to make use of a GPU at all.

The best we can hope for would be affordable dual socket Ryzen machines. But I am not sure AMD would aggressively undercut Intel there; they might rather enjoy high margins similar to Intel's.

Mandelbulber has net rendering, and I think it can be run headless.