Will it run? Yes. The PCIe standard is backward compatible. However, make sure your PSU meets the needs of the new GPU...AND... don't expect miracles as your system will not be balanced with the older components and the new GPU.
Kind of like having a huge engine (GPU) with a tiny carburetor (CPU) in that the CPU won't be able to get enough fuel (data) to the engine for it to realize its fll potential.