The GTX 760 might be OK for medium detail. If you get performance issues, maybe choose lower settings and/or resolution. In older Red Engines (TW2), one of the main factors affecting performance in mid and lower tier cards was resolution. Try something like 1280x720 or so.
Regarding CPU's, I used to have a quite old Phenom II X6 1090T OC'ed to 3.7 GHz and it ran TW3 surprisingly well with a GTX 970. More specifically, ultra detail for everything but high for shadows, grass density and view distance, with a minimum of 50+ FPS inside Novigrad. In the woods and villages, it was faster. This included Hairworks ON (for Geralt only, probably...?). Without Hairworks, I'd get almost constant 60 FPS even in Novigrad. I now have a 6700K and DDR4 (still GTX 970) and yes the game runs much smoother in general (used to have some mini stuttering and so on) and at a somewhat higher framerate (60 FPS, mid-level Hairworks detail), but it's not overwhelmingly faster. So don't expect massive improvement if you replace just your CPU.
In general I think modern games benefit from enough *sufficiently fast* cores (my OC'ed 6-core Phenom was surprisingly adequate) coupled with a GPU that can output your graphics needs. Unless you have money to spare or like me have to buy a computer from scratch, or if anything below constant 60 FPS in Ultra detail is unacceptable to you, don't go around buying a whole new system just for one game.