either CPU will be bottle necked by your video card. over clocking your current CPU and a video upgrade will be a better upgrade. Also you did not post your PSU but a good 400-500w unit is always a good upgrade.
I agree, especially since upgrading to an Athlon II x4 would require a motherboard change. Even an upgrade to the core 2 quad might require a motherboard change, depending on your motherboard support.