For context, I've been getting some old Windows versions working with GPU passthrough thanks to MattKC who has been writing tutorials about it on his forums. I was able to get a Windows XP guest to work on this rig by passing through the 750 Ti, with the RX 480 doing host duties.

It's not as simple as that, the NVIDIA drivers at the time still detected VMs so you need to work around that, and I had to move the virtual PCI-e ports around otherwise there's a conflict between the GPU and the virtual AC97 sound card, and you need to install the F6/floppy drivers for ICH6 so XP can use the virtual SATA ports. But once it's set-up, you've basically got a pretty powerful XP virtual machine. Wanna get Vista and 7 working with this in the future too.

Fun fact, you can run the original GeForce Titan on Windows XP. NVIDIA's last XP-compatible drivers were in 2016.