What is the wattage of your powersupply?
I think the recommended minimum for this is 500 watts, I had no problems running two ASUS 6850 in crossfire with a 600 watt PSU. ( I also OC'd them)
you dont need a 500w psu to power a 6850. you could pull it off easily with a ~380w area. whats most important about powering a gpu is the amperage on the 12 rail