a review site posted the power usage of two 4870's in normal crossfire (cant remember the sites name), it showed that the cards used around 480w under full load. the 4870X2 will probably consume alittle less given that its on one board. Probably around 450w under load.
If your PSU can produce that then your fine. Coolermaster is a good brand in my book, but i do think you may need alittle more. maybe a CM Realpower 650W just in case.
i think it will. the Crossfire configuration is on the card itself and not on the motherboard(like normal croofire). if you Want to Xfire (two 4870x2 in crossfire) then you will need either an AMD Chipset or one of Intels inhouse boards which support ATI
As far as the 680i board, its not a problem, itll work flawlessly. The chipset is nVidia, but the x2 is internal CF, so it actually works within itself, and not across the mobo, so it doesnt require a ATI/AMD/Intel chipset. If, however youd ever want to use 2 cards, then youre stuck with nVidia cards. If youve got the 750 corsair, that also should work fine, and youll have 1 helluva setup