As the other poster said, any NVIDIA GPU would do. You probably want the latest architecture (Pascal), and check what PSU you have and what PCIe power options i

As the other poster said, any NVIDIA GPU would do. You probably want the latest architecture (Pascal), and check what PSU you have and what PCIe power options it has. Depending on that (no PCIe power, single 6 pin, dual 8 pin, ...) you can see how far up the range you can go and still have the card fit.