I would not spend the money to go from a 6th gen setup that needs a new motherboard/RAM/CPU and clean OS setup. I would just get an i5-6600 or i7 for the motherboard you have and get a better card than the RX580 for the money. The nVidia 1660 Ti is a good buy for the price and is a good deal faster than the RX 280. Should get to good quality 1440 gaming on it.
Yes that would be a worthy upgrade. The 9400 is a much better cpu and the rx580 is a really good value right now. What psu do you have? 580s are power hungry. Also mix and match ram isnt always advised so id try to get another identical kit. What kit do you have now.
I would not spend the money to go from a 6th gen setup that needs a new motherboard/RAM/CPU and clean OS setup. I would just get an i5-6600 or i7 for the motherboard you have and get a better card than the RX580 for the money. The nVidia 1660 Ti is a good buy for the price and is a good deal faster than the RX 280. Should get to good quality 1440 gaming on it.