PSU seems a bit on the low side for SLI, if you were going to do that down the road. I would recommend against it, as Kepler should be a lot cooler running. I would think it easier to pick up something ~750W and that should give you headroom for SLI 28nm cards.
900W is OK for two stock 580's, but I'd want more for overclocked cards.
If you are thinking of SLI now with OC cards, I'd go for a round 1000W.
If you are thinking of SLI'ing "Down the road" you would be better served getting next gen cards, since they will use less power, and selling your 580. In this case, I would get a 750W PSU, which is more than enough for 1 overclocked card, and should be enough for 2 "Next Gen" cards in SLI.