It is most likely the CPU, motherboards only really bottleneck in overclocking scenarios. As long as it has a full PCIe x16 slot, performance is generally the same. Your CPU is old, you should upgrade it to something with more cores, a newer architecture, and a slightly higher speed. Depending on the game, Nvidia also has PhysX and other features of the card that the card mostly handles, but the cpu may assist with, further putting stress on your ages CPU.
Are you sure it could be the CPU? I mean, upon googling my own motherboard, it could be, if not older than the CPU itself. The Dell Inspiron 570 Stock Motherboard is a very basic motherboard.