800w is apparently the recommended minimum requirement for 780's in SLI. So the 850 would be fine. Out of personal preference though I would go for a 900 to 1000w. But thats just me due to me always wanting some headroom on the PSU. The one you posted has a platinum rating and looks good based on what I've seen. It will run what you posted.
For a system using two NVIDIA Reference Design GeForce GTX 780 graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 850 Watt or greater system power supply that has a maximum combined +12 Volt continuous current rating of 56 Amps or greater and that has at least two 6-pin and two 8-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!!Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical factor.
Overclocking of the CPU and/or GPU(s) may require an additional increase to the maximum combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock being attempted.
The Enermax Platimax 850W (EPM850EWT) power supply unit, with its maximum combined +12 Volt continuous current rating of 70 Amps and with six (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with two NVIDIA Reference Design GeForce GTX 780 graphics cards in 2-way SLI mode.