Cheapest and easiest improvement would be to buy an i7 3770 or 3770K and drop it into your current board. Since you have a Z chipset, you can overclock. Even the non-K 3770 can be overclocked by 400mhz.
If you want a bit more, an i7 7700 paired with a cheap B250 board is a good value. Alternately, AMD's Ryzen 1600 paired with an AMD B350 board could be a good choice, but in games it probably won't be much, if any faster than an i7 3770, and cost a lot more overall.