The GTX 1060 6GB would be the best choice in that budget. Since that PC looks old, I've included a new PSU. BTW, which PSU do you have currently?
PCPartPicker part list / Price breakdown by merchant
Video Card: Gigabyte - GeForce GTX 1060 6GB 6GB WINDFORCE OC 6G Video Card ($304.98 @ Newegg)...