i swear feznz you are trying to push the idea that he is having the EXACT problem you have, and the PSU is not the problem, sli gtx 580 is about 550w + CPU around 130-145w or OC around 200w and say other peripherals, thats around 700-750w and is enough with an HX850, and corsairs psu can withhold more than what they say, his PSU is fine.
In any case, i would either suggest that maybe take off HDD, get a 140mm for the front, a 20mm thick fan on the side.
But its because your MGPU are too close to each other, either you find a mobo with a big space in between.
If you also have budget, watercooling is an option, but probably last resort if the temp are still not to what you expect.