If that system is based on the first generation of Intel Core I series processors, I probably wouldn't go any newer than the GTX 7xx series graphic cards. Probably wouldn't want to waste money on anything more than a GTX 760 anyway.
I would only get a mid-range graphics card or better if you will upgrade the rest of the system soon. Low end or even an older mid-range graphics card is what you need if you want to keep your system. Even then a better graphics card will still get you better graphics and if you even out the load enough which means Ultra settings and games that are mostly GPU demanding as well as 4k resolution you can get great results even on an old CPU.