Go for the 580 if it's going to last you 4 years. At least then you can shove another 580 in if games start to lag on high settings.
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.
So should I keep 2 560s see what's good/new in 4 years time and look if it's worth upgrading?
Want better/same as than 580 performance off the bat.
Unless it's not worth it.