so adding 200W would allow the PSU to run cooler, and fan run quieter.
I would reason, that if you just get a silent PSU, you're off good enough. As I said, dissipating 50W with a 120-140mm fan shouldn't require a lot of noise. The Dark Power Pro 550 for example should be inaudible at 450W.
Not to mention, that I've never heard a PSU for the last 10 years. GPU's are the prime noise maker in every PC, and a 780 going at more than 250W will make a din, that will drown out any PSU fan. Noise is essentially a non-issue.
And also, that GTX 780 requires around 20A total, if not overclocked. Any of the PSU's I listed will readily provide that non-stop. Some of the more efficient ones lack a DC-DC stage, which will make for slightly more ripple on the signal, but all well within spec.
There's no point in getting hung up over wattage specs, if the PSU is properly engineered. Any one of the PSUs listed is designed to give its indicated wattage 24/7, without exceeding ripple, voltage limits or thermal limits.
Each and every one of the PSU's I listed is a quality PSU. None of them are mid grade. They're merely designed around a lower maximum power draw, and optimised for it, which allows more value-efficient designs, by down-sizing some components that will never see a load where they would be limiting.
Finally, by going smaller, you actually get lower OCP limits, which can help limit damage in case of impending component failure.
The 780 is a card like any other, and has a usable life of around 3 years, then it's going to be too slow for the noise it makes, plus the fan will start to wear out significantly... So no reason to go over the top for that either.
More than 500W PSU in single GPU/single CPU setup are probably only reasonable, if you go full-AMD and OC a 290 that runs next to one of those 220W FX-joke-monsters. But 780 will usually see around 200W, Haswell around 80, 20W for board and disks and fans is generous - so 600W is already 100% overspecced for the usual load. 500W is plenty. 450W is big enough to do overclocks, that take the hardware to the limits and bench at 100%, without exceeding specs. 400W is good enough to have plenty of headroom without an OC. 350W is good enough, if you have a PSU that can short-term provide 110%.
The thing is, the 450W I listed is the absolute maximum possible with that hardware, but OP never suggested an OC. Therefore my suggestion that 450W is plenty. All the PSU's I listed are already well over-specced for the system. Going with even more wattage is a waste.