Overclocked they can do over 300w but I reckon theyll be around 250w. Maybe lower with some undervolt.
You still need to figure out if youre going to cut the powered risers which im assuming youre going to get. If you arent going to cut them then youd need to run all risers on the same psu as your cpu/mobo and that psu need to be able to handle that. This is important, running some risers on one psu and some on another will result in catastrophic failiure unless you cut the 12v feeds on the riser-cable.
Even if you do cut the risers and run 3 full gpus on each psu then Id say 2x750 is on the low side. Especially for the one running cpu/mobo.
I run 5 sapphire 280x dual-x but I use 3 psus.
850w: cpu/mobo+all p.risers+1x280x(6pin+8pin)
750w: 3x280x(6pin+8pin)
400w: 1x280x(6pin+8pin)
Obviously the 400w is overkill but i had that one lying around so figured id use it and save a few bucks. But yeah thats about as far as I would go on 850w and 750w together (both corsair btw).
If youd want two psus and run five cards then I would probably do one 850w and one 1000w and use the same config as I did above except you run four of the cards 6pin+8pin on the 1k psu.
Skimping on power isnt something youll want to do.
Edit: you can see my calculations here:
https://bitcointalk.org/index.php?topic=441927.msg4856476#msg4856476They are on the safe side so you might be ok with less but I wouldnt like to bet on that with my money.
It could work with 2 cards on the 850w I guess, if your peripherals dont draw much and the powered risers dont draw the full 75w from each but like I said. Better safe than sorry.
