Input voltage is the voltage supplied to the CPUs voltage regulation modules, which are now on die. Vcore is the voltage supplied by the VRMs to the actual CPU cores.
Not much of an overclock, but it depends on your cooling and temperatures. 4.4Ghz - 4.5Ghz is not impossible.
Vcore can go as high as 1.35 (it will run very, very hot most likely, water cooling may be needed), Vinput can go as high as 1.9 safely, 2.0-2.2 if you have water cooling.
I run my i7-4770k at vcore 1.3, vinput at 1.8, LLC 1, 4.3Ghz, but my chip is not very good, hits 80C at full load.