IT more or less depends on the resolutions, quality settings, and the like. Overclocking can do quite a bit but noone can tell you exactly what gains you'll see. I can tell you I gained about 15FPS in my favorite games with my pair of GTX550Ti's in Sli overclocked by 10%. Some games won't even react to an overclock on the CPU or Video card while others will take off. So don't expect someone to pop up and just say you'll gain 4.75FPS with 50mhz on the 670. It's not going to happen.
If you want to overclock and find out just what your hardware is capable of? Then prepare yourself. Do the research and learn about it first.