Its diffrent for every card but i whould say 1150mhz is safe.
but if i were you i whould overclock to about 1050mhz and if its stable go to 1100mhz then to 1150 and then 1175 then add about 25mhz untill its to hot (over 80c) or is unstable.
yes,for every card you get different overclocks...if you want the best possible oc,you will need to take the core/memory clock up and benchmark to check for any artefacts....