It was a Quadro M4000 mounted in slot 2 per the Avid Technologies recommended configuration for z840. And it came "on-line", drivers loaded properly during OS install. It was a UEFI machine as well, so not sure how GFX BIOS would have caused the continuing, seeming random problems culminating with a CPU0 L2 CACHE /MCA failure message from POST.
I believe slot 2 is the one HP would have used had I ordered the M 4000 with the machine. Also, I doubt seriously that HP created a machine that could not take a GFX card mounted in any of the PCIe slots that met the card manufacturer's "system requirements". Kinda defeats the entire purpose of standardized buses.