cpu, get i7 3770k and you're fine
also add another 8gb ram, cheap and slick upgrade
Another cpu for that same motherboard won't give you much gains tbh, unless you stream or do something else that needs the extra threads of an i7. To get better fps, you would need to upgrade your whole mobo/cpu/ram to get your fps to the 400's, which would cost over 500€.
Your gpu is already above the threshold that you need with any cpu for csgo. That threshold would be like gtx 1050 ti.
You have to add SSD-drive
CPU ofc, but if also requires a new motherboard and if you go for a top CPU then DDR4 ram with high frequency is a must too xD.
So just going for SSD is alright.