You're pretty much at max for the platform you're running right now. If you want to change CPU to one of the newer gen i5's you would have to change mainboard as well.
That said, the system is pretty good still. Is any aspect of your gaming (or general use) suffering? If not i'd leave it as is for now. Only thing i'd probably do is swap the 580 for maybe a 970 or even a 780 which are comparable in price and performance. Also maybe for convenience upgrade to a larger cap ssd, one that can hold not only OS but games as well.