kamilstoch :
What about the CX series?
Sorry I can't speak for the CX series, I've no personal experience with them. The only review I could drum up was on the CX600 by Guru3d. I like Guru3D, but their PSU reviews are pretty lightweight basic tests. If a PSU passes HardOCP's testing then it's a good PSU. Not only do they load test and torture it, but they take it apart and analyze the quality of the important components used (primarily brand of capacitors) and design layout.
I've gone back to the PSU calculator, and I've made some adjustments to the field of Capacitor Ageing. I originally had it set for 40% to be on the safe side. You want this value high if you plan to keep your supply for awhile. I've done a recalculation at 30% and the recommended wattage for your system is 650W.
The PSU is one place you never want to scrimp on. Its great to have a fast computer, but if it crashes on you two or three times a day or even a week, it takes the shine off your computer pretty quickly. To me one of the most critical components in my computer is the power supply. It's important to go with a reputable manufacturer and have enough power. Also a little research goes a long way. It's not good enough to say well I know Seasonic makes good supplies, because every, and I mean every, manufacturer has at least once made a subpar supply. They also all have their budget lines of supplies, where they save money by cutting some corners in component quality to fit them in a certain budget.
Sorry I'll step down from the pulpit now. From my recalculation, you could maybe drop to a 650W supply, but I wouldn't go any lower and I would be sure that 650W was a good reliable supply. You just won't have any fun overclocking on a borderline capacity, mediocre quality, supply.