Just for fun, I looked up a review of the Asus card:
ASUS GeForce GTX 680 DirectCU II OC | silentpcreview.com
Looks like it doesn't use much more power than a reference GTX680 at maximum load. The 300W capacity of its PCI-E power connectors isn't needed. The two cards together would only use roughly half the capacity of your AX850.
Go for it.
Thank you all for adding your opinions and advice, looks like I'll be going for another card soon.
The Ax series are basically a Seasonic X series PSU.
These are very good Power Supplies (Gold rated) and the advertised wattage on these units are typically its continuous output.
While not all PSUs may be able too, a AX750 or X750 should be able to run SLI 680s without much issue, although it may get tight if Overclocking them.
And have no issues with SLi 570/670s.
The 750 variant can peak around 1K output for a short time.
An AX850 will have additional Headroom and should run SLI 680s without any worries.
You'd be OK even at 750 if not OCing.
With a AX850, I wouldnt think twice about it. It'll be fine.
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