Are you running a graphics card currently? If so what is it and most importantly what power supply model and wattage are you running?
Since you're looking for improvements, RX 580 will still perform decent enough with that cpu, just lack of threads will hurt in recent titles.
If your power supply isn't adequate or is of poor quality i wouldn't recommend RX580, unless you upgrade psu as well.
Second hand i7 6700, they don't cost much, would boost performance quite a bit. That's if you play games. Something to think about.
What's your ram config?