flong777 :
"PSUs run more efficiently under load. IMO, the trend has been to overkill on the PSU when it's not really needed. In reality, when you actually add up the max loads of all components a 350-450W PSU is more than enough to run most single GPU gaming builds."
Vertex, you are right only to a point - PSUs run more efficiently up to about 50% capacity, then their efficiency declines. After that they start to heat up and go to high fan which makes a LOT of noise. For that reason a 350W - 450W PSU is not the best choice for a single GPU gaming computer. Yes the system will work and probably it will not harm the components but it will not run efficiently and it will probably run hot and noisy. A single GPU system can draw 300W - 400W at max and so a 350W PSU is NOT a smart choice. Remember that there are other components besides the GPU and CPU and they all draw power.
To upgrade to a larger more efficient PSU, the cost difference is usually $50-$75. Right now on Newegg, the Corsair HX 850 is on sale for $129.00 after rebates which is an absolute steal. I have this PSU and I have never even heard the PSU fan turn on it runs so efficiently. Even when my system is drawing just 200W, HX 850 is over 90% efficient.
It is frustrating to me to see people downgrade to cheaper PSUs when the cost difference is so minimal. Some people simply can't afford the extra $50 and I get that, but most people can. For those people who can, it is a far superior set up, even for a single GPU>
Look, I never said there is no place for an 850W PSU. You were the one who said "why on earth would you buy a 360W PSU? I just don't see the point." When there are plenty of cases where the 360W Seasonic is the better PSU for the application than the 850W scenario. And yes there are scenarios where the 850W is better than the 360W (i.e. a dual GPU system with an OC'd AMD cpu). The point is that higher Wattage is not always better.
You say it's frustrating to you to see people downgrading to cheaper PSUs - I never said anything about "cheaper". There are plenty of "Cheaper" high wattage PSUs. What I think is a bigger problem is that people who are ignorant to real system power requirements just think a higher wattage PSU is simply better, when it's not.
In fact, "Wattage" isn't really what counts. It's Amperage for each voltage level that you need to power the components. Here is an example. Take this Cooler Master 525W PSU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817171065
If you look at the label, it has split 12V rails with only 20A each. Usually it will have one rail supplying the CPU and the other supplying the PCIe connectors. The Seasonic happens to have a single larger 12V rail with a full 30A capacity. You'll probably be fine with the 20/20 split with the CM, but with other PSUs, you might have to be careful.
A more extreme example is this cheap ass "600W" ePower PSU.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817101048
Even though it's rated for "600W" (don't ask me how they get that rating), it only has 33A total available at 12V. So that's only 3A more than the 360W Seasonic, even though it's "rated" for 240 more Watts.
So the real problem hear is that people who don't know any better just buy the PSU with the greater "Wattage", when that is not what really counts.
Your 850W Corsair is a great PSU. But quite frankly, it's overkill for Most situations, and money spent would be better spent on other components, or just not spent at all. Note I didn't say "All" situations, because it definitely has it's place. But at 200W, I'm afraid it does not get higher than 90% efficiency:
http://www.legitreviews.com/corsair-professional-series-hx850-power-supply-review_2037/7
In that situation, drawing 200W, you'd be much better off with the 360W Seasonic. This is because the drop-off in efficiency below 50% power is usually much sharper than the drop off above 50%. The best rule-of-thumb is to size your PSU to be typically 1.5 times your Max system load. This allows the PSU to operate in it's sweet spot of from 50-75% capacity.
Finally, you might actually look at some PSU calculators. You'd be surprised how little power some pretty powerful systems consume. As an alternative, you can just look at most GPU reviews. When they show GPU power consumption, it is usually Total System Power. Here are a couple examples showing total system power under load:
http://www.anandtech.com/show/6276/nvidia-geforce-gtx-660-review-gk106-rounds-out-the-kepler-family/17
http://www.legitreviews.com/powercolor-hd7870-devil-2gb-video-card-review_2244
I'm not trying to say "why on earth would you buy an 850W PSU". It's your money, and there are certainly applications where the 850W is the right choice. Just try to understand that there are just as many applications where the 360W is the better choice for that particular application.