how much power can you get out off your psu? Due to the fact that you bought a complete pc your motherboard is probably kind of shit. I would go for a new motherboard and new ram. You will probebly have to buy a used motherboard since you are using an old CPU. 😀
Intel i5 8600k would be probably be the best if you look at gaming preformance to price. Would also be the most future proof.
NOTE! requires new motherboard and Ram.