Most power efficient CPU

There are celerons, atoms.....

Yup...

Neither of those are that powerful for video processing.

Why not use a more powerful power supply? It's not as if a 350w power supply constantly sucks 350W from the wall, you know.

As for your question, if you want decent power for video processing any of the Intel Ivy Bridge processors are worth a shot. They have a 77W envelope, as opposed to my Phenom II 1090T which draws around 140W (basically double).

They could be...

There are low power i3, i5 and i7s around....

http://www.kustompcs.co.uk/acatalog/info_56019.html

Ivybridge like already said 77W (or lower). AMD options would suck more power. Making a low power HTPC to then use those adobe apps on is a bit of a contradiction though.

http://www.quietpc.com/products/heatsinkacc/ivybridge

i5-3470T is only max TDP 35W. HD 7750 is another 80W? So the 200W is plenty.

You can watch everything Intel has ever made from http://ark.intel.com

^^^

That depends on how many amps the 200w PSU can supply on the 12v rail which the CPU, GPU, fans, hard drives, optical drives draws power from. Using the watts you provided the CPU and GPU uses 115w of the 200w which means they will draw a little over 9.5 amps from the 12v rail. Add in the fans, hard drives, optical drives and the power draw on the 12v rail increases.

I generally recommend a 300w PSU for anything with the HD 7750. 250w if the PSU is high quality like a Seasonic or Enermax PSU.

Here's a good read that compares several of Intels energy efficient models to their standard counterparts (Sandy Bridge only):

http://www.xbitlabs.com/articles/cpu/display/core-i5-25....

Keep in mind that TDP =/= Power Draw

What's interesting is that some 65W TDP chips actually draw less power than 35W rated TDP chips. Give it a read.

Actually I'm doing the same thing here.
With 200 Watts-exactly same as mine, although mine runs on batteries and DC-DC converters:

You will need a laptop processor, QM line of i7,i5s are good and have very low TDP and even lower actual usage relative to others. If that's not enough, you can always undervolt them.

For a GPU, I went with seriously undervolted HD7850. ATM I'm thinking of installing some kind of switch/router to the PCIe riser card to turn it off when it is not being used, ie) DP signal is off.