ok the best in my opinion on such a old system ,,,, i would run is maybe a gtx 1650 i triedrunning a a gtx 1060 6gb on a old core 2 quad 9400 overclocked and it bottlenecked that gpu some, its just that instruction sets and the technology isn't there to keep up with a modern graphics card like well even a 1660 super would probably be too much for it .
but if this is like the beginning of a grander scheme of things and future plans are getting a newer cpu ,,, then thats a different story and bottleneck though it will happen ,,, just means youll get the max performance out of that syem ,, till the new upgrades come into play. so i if thats the case and your on a tight budget ,,, I suggest looking on the used market for a GTX 1070 and get a decent power supply ,,, should be able to get that gpu soon for pretty cheap.