Hi,
I have used a few calculators and always came to a similar result. But Nvida says to get a 800w PSU with 2 780sli
Yes you can completely trust that. Even that may be a little high. It has been proven you can run a GTX 780 and an i7-4770 of of about 450W although it isn't recommended. A good 550W PSU will be fine for your build but I'd recommend 600 or 650W to give you a little room to upgrade in the future.
Just be sure the PSU you pick is from a reputable brand and is 80+ Bronze certified at a minimum.
The companies always overstate their PSU recommendations.
That sounds great, Thanks. I just got 700W Straigth Power BeQuiet! PSU after New Year and I was kind of worried that I made a wrong decision with getting a too weak PSU.
My plan is to get a 780 and then later when they get cheaper to get another one, because I want something that will just work for like the next years, without touching the system again.