Dude I am not sure one can actually over clock Intel Graphics. I could be wrong however. Intel Graphics is designed for the non gamer who still wants multimedia capability. The only way to really improve your graphics is to buy a discrete card. The 560 Ti is popular.
Doesn't make much of difference though, the integrated processing core is made more for idle workload or 2d game processing. if you're going to be playing 3D, expect no more than 30-35 FPS on medium after overclock.
There should be tons of guides if you search google.