with your current CPU, upgrading the GPU wont do much. yes you will get a bit more performance, but your CPU will be bottle-necking the GPU. Get the CPU, Mobo, and RAM first, built a solid foundation for a new more powerful card. Going with a better card now will not net you that much more FPS.
I had a 3rd gen i7 at 4ghz, 32gb DDR3 ram and my 1070. I would get maybe 20-25 FPS on the ground at Nellis with all setting set to low, and maybe 40 if I was lucky away from Nellis. I upgraded to what is in my signature now, but kept the same video card, and I am now getting 45 FPS on the ground at Nellis with about all settings set to high. I have even hit 60+ when away from Nellis. This is all in VR btw. A good, fast CPU, good mother board and RAM will make a big difference in the end.