If you already have that 780, to put to proper work you will need at least an i5 or an FX6300/FX8300. Of course, you can use whatever CPU you like, but anything below that i5/FX will make that GTX to idle a good chunk of the time. Mind you, for 1920x1080 even an GTX770 will do over 30FPS in almost any game at highest settings.