Originally Posted by pharcycle
Personally I'd not bother down/sidegrading... the max TDP of the Q6600 is 105W vs 65W of the E8500 so at most you're gong to save 40W when they're running flat out.
However this is a bit of a paradox... they're pretty similar if not the same architecture so I'd imagine that on idle the Q6600 and E8500 will use roughly the same power. Under load the Q6600 would draw more but then, you probably want it to as it's doing more work.
What motherboard do you have? Can you under volt and under clock the processor as this will save you a heap of power... power varies with the square of the voltage (P=V^2/R) so this can drastically reduce your power consumption
What might be worth while is getting a power meter and measuring what is actually being drawn from the plug as there could well be other things which would offer more benefits. For example a PSU only provides its rated efficiency AT the rated power, below this the efficiency drops off.
Alternatively, some of the dedicated single board computers or media centres would be the lowest power option if a full PC is really just a luxury and you're interested in is media playback. A Rapsberry Pi for example is only 10W and can playback 1080p @24Hz should you want
I agree with lots of this post, I just wanted to point one thing out that I think was missing.
The Q6600 is indeed not using that much more power than the e8400. And yes, both can be undervolted/underclocked to save on power. However, the stock Q6600 clock speeds are already pretty low, while the e8400 is actually 600MHz higher. Since many media players are only single threaded, I think it would be hard to lower the clock speed of the Q6600 very much and still leave it capable of stutter free HD playback. The e8400 would be easier, since you could easily lose 600MHz, and still be at stock Q6600 speeds. But like I said originally, I really don't think its worth the effort unless its really cheap (like, almost, or exactly free).
And good call on the other options if all that is required is video playback, the Pi is a good example, and even some smart TV's can do it themselves these days. So that is something worth looking into.
Either way though, good luck with whatever you do