I have one z400 Series 2 with the 6 slot for memory main board. It is the oldest z400 in the office so will be the next one to replace but need some more video until then for Trimble earthworks software. It appeared that the Nvidia P4000 was going to be what we would use in the (soon to be) new z440 or Z4 and should also be compatible with the z400.
Interesting thing is that it took out the power supply after 3 days. I guessed the power supply was "ready to fail" and replaced it. That one made it 2 days.
Any guesses? Too much power through the wrong connector? PCIe v3 card in PCIe v2 slot?
Interesting, when we upgraded another workstation, my son got the just outdated z400 and being a gamer wanted a gamer video card. Ran a clearly too big 225w card for a couple years that would shut down the entire computer after extensive use but never actually broke anything.