Don't seem to be able to find any concrete information on google on this. I have seen 40 amps posted about on google for a dual HD 6850 system plus overclocked cpu ... this sort of makes sense as 2 HD 6850s probably draw 260watts say 180 watts for OCed cpu that makes 440watts in total. 440w/12v = 36.6a reoccurring. That leaves you with about 3 amps contingency!
What do you think? what is the bare minimum ampage for a mid range xfire setup like 6850s.