Bear in mind efficiency ratings too.
Say your PSU is a 400w job, and your PC theoretically draws 380w at full load. You'd think that would be fine, right?
The PSU would have to be >95% efficient for that to be true. Most of the good ones only hover around the 85% mark, so the max it can really give you is 340w.
I'm sorry but you need to read up on power supplies and rethink those statements as they are not correct.
Take a look at page 4 of this review.
http://www.silentpcreview.com/article656-page4.html
Notice how the 650W power supply is taking in 820 watts from the wall (AC input) and giving 650 watts to the rest of the PC (Total DC output)?
I could link to similar charts for other power supplies but I'm hoping you can get the truth from one example.
You might also notice that a power supply that claims up to 88% effeciency will only do that in a cool room in Europe (or at least not in my part of the US). The reason for that is that power supplies are more effecient when plugged into a 220v wall plug like you'd find in Europe than they are at 120v like you would find in the US.
Furthermore they lose effeciency at higher temps. So if your system is overheating you won't be able to get the rated power out of them. But if that is the case you can just add more case fans, buy a different case, or change your Air Conditioner settings in your house.
It is true that you don't want a 380w load on a 400w power supply but that has nothing to do with the backwards math you are using for power effeciency.
The reality is that the S12-550 Energy+ model can handle a 500 watt load but it will be more effecient and quieter at 200 watt load.
For long term energy usage, reliability, expanability/scalability, it is best to give your PSU some head room. Don't buy the Seasonic 330w when the Seasonic 380w is about the same price. Consider the 430w, 500w or 550w Seasonic if the connectors are needed and you want room for growth.
Just rember quality isn't always directly related to price. If there is hardware out there that won't run on a S12-650 E+ then it is to far outside of mainstream for me to even consider it. And don't assume that if it won't run on some other company's 850 watt unit that it won't run on a Seasonic 650 watt unit.