Even thou the 6870 got 2 power plugs on it, it uses pretty much same power as 6850.
(look up review of both cards and see, they tested power)http://www.guru3d.com/article/radeon-hd-6850-6870-crossfirex-review/4
I have 6870 and only 1 6 pin from my power supply, adapter for second one,
with CPU and GPU maxed at 100% the power load is 350watts for whole system
and i run it on 430 watt PSU, (50-80% load is most efficient)
power draw when playing games is less it jumps from 200-260 watts
I would get single 6870 you can snag one off newegg for same price as 6850 when there is sale as well