Is power DC power supply ground, connected to the chassis earth on a desktop PC?
Yes in a normal desktop PC mains ground is connected to both the chassis and to the ground pins on the power supply output connectors. In turn the motherboard connects both chassis ground and the ground from the power supply to the ground of the expansion slots.

Whether your aquisition card connects the ground of the PCI slot to the ground of it's input connector is a question for NI but I expect it does.