A GTX 1660 Super would make more of a difference.
But if you want to upgrade your CPU look for a cheap used i7-6700 or 7700 (be sure to update your BIOS if you get the 7700) . I wouldnt spend more then 80-100 USD. Anything past that and it would be better to just upgrade your Mobo to something newer like a Ryzen 3600 and B450 mobo. If you do upgrade to a Ryzen platform I would also suggest getting some 3600mhz DDR4 RAM in a dual channel kit as well, 3200mhz will work for Ryzen but 3600 is ideal for best performance.