Voltages

[dionysus68]

I have the xfx 5970 black edition and downloaded the overvolt tool, i managed to get it working on windows 7 64bit. Thing is i'm not sure what voltage to set it at with out overvolting and burning out my card. I want to put the core clock at 850 and memory at 1200. Has anyone been able to get it above this clock speed and if so, with what voltage? I am also considering adding an xfx 5870 and try for a 3 card set-up, can i crossfire these cards or do i need to get another 5970?

My mobo is the rampage 2 extreme, 12gb ddr3 dominator 1600, i7 920 clocked to 4mhz, 1050watt enermax psu and, windows 7 64bit.

Any advice would be great......