Different cards will overclock differently. I dont know the stock clocks but what you should do is just move each up by 5mhz and run a test like 3dmark untill you start getting artifacts.
we cant tell u that, u have to try it for yourself, watch the tempterature when overclocking and make sure u dont get any artifacts, if u dont know what they are then i highly suggest u read an overclocking guide before attempting this