How far gpu oc with this gpu?

hi guys i have this prebuilt computer from bestbuy. i was wondering.... how much of an overclock should i do on my gpu with out exceeding my psu's limits and or increase the voltage on the gpu.

so first off i have an amd radeon 7770 2gb gddr5 and my psu well the brand name is unknown but its 460w most likely 80plus bronze :P i have tried +70mhz on it b4 i knew what my psu was but since i do know now... i have downclocked it to default clock speeds at 1000mhz.

im just here to ask you what my limit should be if i want to overclock my gpu

NOTE: only reason im asking is cuz i wanna squeeze a couple extra frames from bf3.

someone plz reply? bump...