Sorry about the wait...
temps on that card can get a little high so id suggest a modest 875mhz core and a 1000mhz mem.
Safe temps are around 80'C but don't let it get above 100'C
Tops for that card would be about 1000MHZ & 1100MHZ (Water cooling would be required) Dont try pushing it over 875 with stock cooling
Check out this linkhttp://www.overclock.net/ati/633816-how-overclock-your-ati-gpu.html
Edit = I have no idea what the core voltage would be... not all cards are created equal, some need a little more juice to get themselves stable