Anyway getting back to what I was saying, the difference between the energy efficient 4600+ and the E6300 is very small at only 6w, but that's for the entire system in the test setup over at xbitlabs.com. If you only take a look at the CPU usage while at idle it's 14w for the Athlon 64 X2 4600+ vs. 26w for the Intel Core 2 Duo E6300. That's only a difference of 12w.
So how much will 12w cost you over the year? Well if you live in NYC, you'll be paying $0.19 per KWH. Being on over the course of 1 year under 24/7 conditions that comes out to about 105KWH, and translates to $20 for the entire year. If you live in an area where electricity is only $0.07 per KWH, then that works out to less than $7.50 per year.
In the grand schemes of things $7 or $20 over the course of 1 year doesn't amount to much for the typical person.
Alright, what about under load? The following is a continuation of the xbitlabs.com article I linked to before:
http://www.xbitlabs.com/articles/cpu/display/amd-energy-efficient_8.html
As you can see the Core 2 Duo E6300 CPU consumes only 44w underload, while the Athlon X2 4600+ consumes 75w. That a difference of 31w in favor of the C2D E6300. Looking at the test systems, the E6300 PC consumes 218w of power vs. 268w for the energy efficient X2 4600+ PC. That's a 50w difference. So much for AMD's energy efficient CPUs.
You best choice is the C2D E6300. Period.
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If you really want an energy efficient HTPC, then you should really start with the power supply. The PSU's efficiency determines how much power is being wasted by the PC. As a very simple example, suppose your PC draws 200w total for all it's components. PSU #1 is 65% efficient. PSU #2 is 80% efficient. That translates to the following:
PSU #1 Total Power Draw = 200w / 65% = 307.7w
PSU #2 Total Power Draw = 200 / 80% = 250w
This means PSU #1 will waste 107.7w of power which gets converted to heat. PSU #2 only waste 50w of power.
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The next place to look at is the GPU value cards tend to draw less power than high performance cards. Below is a link to another xbitlabs.com article, but this one is for the GPU:
http://www.xbitlabs.com/articles/video/display/power-noise.html
Looking at the various cards it should become obvious that ATI Radeon GPUs draws more power than the nVidia GeForce series. If I were to buy a GPU right now for an HTPC and I wanted to do some gaming, then I would get the GeForce 7600GT:
http://www.xbitlabs.com/articles/video/display/power-noise_5.html
This is a "performance-mainstream card" which only draws 35.8w; more if overclocked. Compare that to the "mainstream cards":
http://www.xbitlabs.com/articles/video/display/power-noise_6.html
Note that the Radeons are less powerfull, but draws more power. Also note that the 7600GT draws less power than the less powerful GeForce 6600GT.
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Hope that helps. But it should be clear that you want a C2D E6300 or maybe the E6400. And you want to pair that up with the GeForce 7600GT, or maybe the 7300GS if gaming is low on your list of things to do. The 7300GS only consumes 16.1w of power.