Sorry but 850W will not cover 2 x 580's SLi.
http://www.anandtech.com/bench/Product/308
EDIT: Alright then. According to the nvidia website the 580 takes a max of 244W which I highly doubt but based on that, two of them should be fine under that Lepa 900W.
244W doesn't make sense in my opinion but if thats what nvidia says, you have proof to back you up if anything goes wrong.