It would appear that for some reason the PCI slot has problem, since the device manager did not list any GV800 devices (there should be a few under Sound, Video and Game Controllers), and one of the the "PCI Standard PCI-to-PCI bridge" under System devices is labeled with exclamation.
Solution: swapped with another PCI slot. Windows re-detects everything, installing drivers (I had to get some missing drivers from Windows/System32/Drivers/). It works. Swapping back to the original PCI slot also works now.