I can spend $300 to $400

You will need a CPU if you want a better GPU. Also, what power supply do you have? If you have the stock PSU you will probably need a new one.

What about,

i7-2600 or 3770 *Actually, you will need to double check CPU compatibility. I cant' immediately find support for that mobo)
RX 580
PSU within budget
 


 


 


 
I think I going to buy a new computer the medion erazer x67011
It's got i7 8700
Geforce gtx 1060 6gb
The motherboard is a Z370
Can the this computer support the new rtx range or any thing better in its gpu