All a piocPSU really does is convert 12VDC to the various lower voltage lines needed by a computer. That's it.
So it needs 12VDC to start with -- hence the AC/DC adapter which gives you 12VDC. Part of the 12VDC from the AC/DC adapter is simply fed right through the picoPSU into the motherboard (and peripherals), the picoPSU is only responsible for the lower voltage lines.
It's like splitting a standard ATX12V PSU into 2 parts -- one part for 12V and another part for the rest.
As for power ratings... it gets a little bit tricky, but since the majority components in a modern computer is powered via the 12V line, the AC/DC adapter's rating (and efficiency) is far more important than the picoPSU's.
The 80W picoPSU can handle 4A (steady state) through it on the 12V line; that's 48W. So the remaining 32W is on the other lines. If you match the AC/DC adapter's rating with the picoPSU, the above #s are the most you will get to your system from the pair. If, on the other hand, you go with... say a 100W 12VDC adapter, that will give you higher capacity for peaks, though the picoPSU could now be potentially overloaded (and burned) if your computer demands more than 80W and the AC/DC adapter delivers it. So ideally, you want to match the picoPSU power rating with that of the adapter.
Is an 80W picoPSU (and 80W adapter) enough for your setup? Probably. The CPU will not hit 35W in normal apps, esp. video play, my guess is than in normal use, the CPU + mobo + HDD will pull 45~50W max together. The Bluray player could pull a fair bit, esp. when the drive ramps up or maybe during burning... but it's usually brief, and even at max, I doubt it'd be >15W.
btw, don't be fooled by the 96% efficiency cited for the picoPSU.
This is only for the picoPSU itself. In other words, when it converts 3A of the 12V line (a total of 36W) to 5V, 3.3V, etc, it loses only 4% of that power -- which means it outputs 34.56W. But the total efficiency is the combination of picoPSU & brick. So going back to the 3A example, if the brick's efficiency is 85%, then to deliver 36W at 12V to the picoPSU, it has to pull 42.35W from the wall. The real efficiency is 34.56 (final output power) divided by 42.35 (AC input power)... or 81.6%.
A further complication -- the 81.6% applies only to the voltages the picoPSU converts, not the 12V line, which is passed straight through. The efficiency on the 12VDC is determined solely by the adapter... which in my example, is 85%. Since much of the power to the computer is on the 12V lines, the additional loss through the picoPSU is fairly trivial; the efficiency of the adapter is far more important.
Practical conclusion: Buy the most efficient brick you can find of the same rating as the pico. See this PDF doc (at Energy Star) on power brick efficiency. http://www.energystar.gov/ia/partners/p ... otocol.pdf
Basically look for the ones marked "V" -- looks like the best are ~87% efficient.