The lowest wattage PSU I see on the list at SLI Zone
is 950W, so I'd be skeptical that 850W, especially that 850W PSU would handle it well.
It's possible it will work, but I wouldn't bank on it.
That GameXStream PSU you have linked does not appear to be 80+ certified. If you end up buying a new PSU, make sure it's 80+ certified and has Active PFC.