O'clocking msi hd 6770 with afterburner.

actually i'm o'clocking from about a year...I've overclocked many processors through bios but have been my msi hd 6770(through afterburner not AMD overdrive) from about a month ..i get 5 options in the afterburner
1 )core voltage(mv)
2) power limit (%)
3) core clock(mhz)
4)memory clock(mhz)
5)fan speed(%)
out of these the first two r unchangeable ..but today i just got the 1st unlocked in the settings but i don't know what it does..so my 1st Q is what is this core voltage is it dangerous to change it , will it effect the cards performance if i increased it & how much should i increase it
the 2nd one is still locked i don't know how to unlock it.. if changing it increases the performance I'd like to do it..so plzz..tell me how..

i then changed the skin of afterburner to legacy & i got an extra option SHADER CLOCK (MHZ) instead of the power(%)..but its also locked & same Qs about it...