YES it is.
Generally all gpus are compatible with a mobo. What you need in the motherboard is a PCIE x16 slot, and recently I have learnt that you donot need a x16 3.0 slot to enjoy the full benefits a x16 2.0 is also fine. All mobo have pcie x16 slots
they mean what number of graphics cards you can attach in parallel of nVidia and AMD respectively. You can either have multiple cards of nVidia or amd and not a combination of both.
You need to have a power supply with enough wattage to run these computer components. You need to check these when buying a card. You will need a 750watt power supply to be on the safe side although a 600watt will also do.