Depending on the exact GTX780 model, actually the 290X may be running less hot than the GTX780... at least the reference model exhausts outside - unfortunately most Nvidia aftermarket coolers exhausts inside the case... which ultimately causes too high temps not only on the VGAs but also on the CPU and other components too...
For that matter, the Corsair 800D is a very unfortunate choice for air-cooling those systems with inside exhausting VGAs...
860W for GTX780s in SLI is enough only if you run them without OC, or very moderate OC...
Obviously it depends on what are you gonna use your system, but for any SLI or CF I can not recommend sub 1000W PSUs - but I'm only overclocking and benchmarking... starting from the GTX 4xx series and HD 5xxx series cards, a single 1250W PSU was always the bottleneck of my max OC'ing when I was playing with 2 or more cards...
Don't forget, to really enjoy a graphics OC, you do need to have your CPU and RAM OC'd too - and those need some juice for that... in return they will produce more heat and you need more cooling, which also needs more juice...
For everyday use/not so demanding applications none of the above mentioned graphics cards will run at their max, so they won't need a max power draw either...
As I said before, it's about what you're gonna use your system for...
For that matter, the Corsair 800D is a very unfortunate choice for air-cooling those systems with inside exhausting VGAs...
860W for GTX780s in SLI is enough only if you run them without OC, or very moderate OC...
Obviously it depends on what are you gonna use your system, but for any SLI or CF I can not recommend sub 1000W PSUs - but I'm only overclocking and benchmarking... starting from the GTX 4xx series and HD 5xxx series cards, a single 1250W PSU was always the bottleneck of my max OC'ing when I was playing with 2 or more cards...
Don't forget, to really enjoy a graphics OC, you do need to have your CPU and RAM OC'd too - and those need some juice for that... in return they will produce more heat and you need more cooling, which also needs more juice...
For everyday use/not so demanding applications none of the above mentioned graphics cards will run at their max, so they won't need a max power draw either...
As I said before, it's about what you're gonna use your system for...
