i was wondering if anyone knew a good core and memory speed for 2 xfx double dissipation 280xs in crossfire? i dont game with a closed case so i can get extra air, i have an xfx core 850w psu. also please tell me how to increase them ammount of voltage i give to each card in amd overdrive. also how much voltage i should give them for the stable clock/ memory speeds you give. thanks.