Actually, if you're planning on going sli in the future, it might be best to stick with x58. From what I've read Sandybridge still has the same pcie limitations for graphics that P55 does. (Although I have seen that x8 x8 sli isn't that much slower than x16 x16)
As to your question about whether the ax850 can handle two 580s, check this review
http://www.guru3d.com/article/geforc...-sli-review/14
Their system uses 719W at max load so it should be okay, you just wouldn't have a lot of headroom after you upgrade.