the memory of the motherboard is independent to that of the gpu. as long as the slot is pci-e and the gpu is pci-e it should work (of course your psu should be able to handle the power requirement)
they are independent (in terms of compatibility), otherwise we would not be able to use GPU's with (g)ddr5, since we only have up to ddr3 yes? (ddr4 is coming i believe)