If you do not Overclock then yes you can. If you do Overclock then no, you will most likely surpass the PSU's limitations.

Even if you didnt OC, personally I would not be 100% comfortable running 2 580's on an 850 watt psu... Bare minimum, I would go 1000w, optimally 1200. Will it run it possibly? Yes. But you would be so close to the limit it would not be funny, and you are not supposed to run at more than 80% peak power for extended periods. Get something a little beefier and let em fly!

The Corsair AX 850 is an extremely solid PSU and would not have much issue in running a SLI GTX580 system even if it's overclocked.
 
Look here:
http://www.guru3d.com/art...-gtx-580-sli-review/14
 
A overclocked X58 system with 2 GTX580's tips the scales at just over 700w at full load.
 
The Corsair AX850 PSU is rated to deliver 850w continuously for the operational lifetime of the PSU. You should be OK with your current PSU as your sandybridge P67 based system consumes evel less power than the above tested X58 system.

thinkgreenn

Thank you very much for all the comments
@crptacularOne

So the extra fans/pump etc wont bother?

If i do get the second gtx580 i will be making my system a dual loop by adding a second mcp355

I dont want to be cheap and not buy a 1000+ psu. If its really needed i will buy one but if the ax850 can handle it i dont see why i would buy a new one.

Also where can i find the estimate howmuch power the z86 uses with and overclocked 2600k

Kind regards

Note, that power draw is with the GPU at stock clocks. I would also note that while it's certainly possible to run a quality PSU at or close to it's rated maximum continuously, I don't know why anyone would just because of the loss in efficiency and the heat generated. My personal preference is to size a PSU at 80% of your expected maximum power draw. For efficiency, lower temps, and as you've cited, being able to upgrade equipment. Personally, I think you'd be better off with a quality unit of 1000 watts and leave yourself a decent amount of headroom.

thinkgreenn

Thank you very much for all the comments
@crptacularOne

So the extra fans/pump etc wont bother?

If i do get the second gtx580 i will be making my system a dual loop by adding a second mcp355

I dont want to be cheap and not buy a 1000+ psu. If its really needed i will buy one but if the ax850 can handle it i dont see why i would buy a new one.

Also where can i find the estimate howmuch power the z86 uses with and overclocked 2600k

Kind regards

 
Here you can see a EVGA Z68 FTW board with a 4.9ghz 2600K tips the scales at just over 280w at full load on all cores and threads.
 
http://www.guru3d.com/art../evga-z68-ftw-review/10
 
Though I think that board consumes more power than your ASUS board. Here is an ASUS Z68 board at ull load and it only tips the scales at 263w  at full load:
 
http://www.guru3d.com/art...iv-extreme-z-review/10
 
I really think you're going to be OK with your Corsair AX850w PSU. It is a very good platform and more than capable of delivering it's 850w continuously and barely affect it's efficiency rating. Here you can see that even at full load it maintains nearly an 87% efficiency rating.
 
http://www.jonnyguru.com/...ile=print&reid=197
 
Unless you really, really want to buy a bigger PSU, I don't see a "need" for it in your system.