Generally speaking, AMD CPU's are not as efficient as far as performance per watt goes.
As far as trying to search out the lowest rated power for each part at any cost goes...that is wasted effort/lost cause IMO. Pretty much any configuration for a modest HTPC using integrated graphics, whether it be Intel with it's IGP or AMD with it's APU should run in that 30-60w range these days as it is with current parts, with no particular attention paid to efficiency.
The only 3 areas that would really be significant are the CPU, Video, and Power Supply.
CPU....pretty much common sense here. Keep in mind the purpose and stay away from the overkill. You don't need a power hungry A10, or a juiced up overclocked i7 for a HTPC. Stick with a lower end modest processor which by nature consume less power. A general HTPC isn't processer intensive, so anything currently out there short of an Atom level processor will do everything you need just fine.
Video...same idea as CPU, but there can be benefit to a powerfull card. You don't need some powerfull HD7970 or something for an HTPC. Out of the current choices out there, even the lowest integrated video has enough hardware support for a basic HTPC. In your case however, you do want to pay attention to the 3D capability since you intend on using that. Some of the lowest models do not have that functionality, which appears you already know. Where better video cards come in is if you want to do significant postprocessing/scaling/etc with codecs like MadVR....which can use significant GPU resources. If you don't intend on using something like that, you don't need a fancy vid card and the power that goes with it.
Power supply...the overall wattage doesn't matter much as long as it is sufficient. It's all about efficiency as renethx started explaining. That said, there is more to it then simply 80+, bronze, gold, etc. Peak efficiency occurs at a certain load particular to the actual PSU and can vary from one model to the other. For example one particular power supply could be listed as 80+ and is in fact 80+ efficient at say the 70-90% load range, but only be like 70% efficient at the 10% load range. You might have another that is not 80+ or anything that is actually like 85% with a low load but much less with a higher load in relation to it's rated output...that sort of thing. You can find efficiency charts for some higher end higher wattage PSU's that show it's efficiency across the whole load range, but isn't all that common and you probably wouldn't on lower wattage more modest ones, so there is no real way to know for a modest power supply for a low power system. I would simply go for the highest quality 300w you run across that isn't outrageously more expensive then the low priced ones with similar wattage as far as ATX goes...and as far as the ITX types, get a seasonic since there is them and cheap no name with no other option pretty much. Basicly what I am getting at is there is no reason to spend $100 on a super duper best most efficient 300w that money can buy vs a decent $30 one just to save literally a few dollars per year on efficiency.