I must have pissed someone off, or, I was just exceptionally lucky. After struggling with half a dozen weird Windows issues, trying to roll back with system restore, failing, trying to upgrade to Windows 10 AU, failing, then succeeding, then having more issues, then scanning for viruses...
It turned out to be bad DIMM. Seems unplugging all the RAM (twice) to move everything around in the case to first make room for the 1080, then to give it more breathing room and try to cool case better, plus the exploding PSU... fried one of the DIMMs. Of course, this was determined after countless hours of troubleshooting and frustration. I knew going to 4K was going to be disruptive, but HOLEEE SHEEET.
Also, just for fun, Memtest86+ 5.01 will hang your computer on test 7. Bad CPU? Bad MB? Nope, fear not, turns out SMP memory testing is in beta (has been for years, I guess, as memtest86+ hasn't been updated in years), and on some CPUs and configurations when using SMP testing it will hang your computer. You know, just give that nice, extra layer of JFC WHAT ELSE CAN GO WRONG?!
System seems stable on 12GB and I got another stick on order.