There are three ways to decrease power consumption.
1. Replace the power supply (PSU) with one that is 80PLUS certified. That basically results in a minimum efficiency of 80% across all loads. PSUs that are not 80PLUS can be as inefficient as 60% depending on the load. 80PLUS PSUs are rated from Bronze up to Platinum; 80PLUS Platinum PSUs are up to about 91% efficient.
Assuming in total your computer needs 150w went idle, that means a non-80PLUS PSU may use as much as 250w from the AC outlet (150w / 60%) if your PSU is only 60% efficient. Or about 214w if your PSU is only 70% efficient (150w / 70%). An 80PLUS Bronze PSU that is 80% efficient means the PSU will draw about 187.5w from the AC outlet (150w / 80%). An 80PLUS Platinum PSU would lower that to about 165w of power (150w / 91%). The excess power (beyond the 150w) is generally wasted mostly as heat and a tiny, tiny bit as noise.
2. Replacing the CPU can help lower power consumption, but at the cost of performance. That said, based on what you want to do the drop in performance should not be too noticeable. The Q6600 consumes about 36w of power while idling and 87w under load. An option would be to replace it with the dual core E6600 that idles at about 21w and uses 52w of power under load.
From:
http://www.xbitlabs.com/articles/cpu/display/core2extreme-qx6700_11.html#sect0
From:
http://www.xbitlabs.com/articles/cpu/display/core2duo-shootout_11.html#sect0
Under load the Q6600 consumes 35w more power than the E6600 under load. However, watching a movie, or recording a TV show will not max out the CPU so it unlikely the Q6600 will use 87w in your situation. Your HTPC will probably be idle a good deal of the time so the power saving is only 15w under those conditions. While difficult to estimate, going from a Q6600 to an E6600 will probably save you on average of about 25w of power under mixed conditions.
3. Replacing the video card can reduce power consumption and it is possible to have a video card that is just as powerful as the old video card, or even more powerful. First and foremost, do not buy a Radeon HD 6xxx series video card. You have an old motherboard that only has a PCI-e 1.x slot. All Radeon HD 6xxx series cards are PCI-e 2.1 cards. There can be compatibility issues inserting a PCI-e 2.1 card into a PCI-e 1.x slot. The initial Radeon HD 5xxx series cards were PCI-e 2.0 cards (no compatibility issues with a PCI-e 1.x slot), but later Radeon HD 5xxx series cards were PCI-e 2.1 cards.
I would generally recommend a Radeon card vs. a GeForce card because they generally consume less power. I would also recommend a Radeon HD 5xxx series card (PCI-e 2.0 of course) vs. the older Radeon HD 4xxx series card because of the lower power consumption. The Radeon HD 3850 uses about 63w of power. As a point of reference, the HD 3850 is less powerful than the GeForce 9600GT. It is probably as powerful as the Radeon HD 4670.
I recommend buying a PCI-e 2.0 HD 5670 card. It is more powerful than the GeForce 9600GT which means it is more powerful than the HD 3850. It also consumes about 29w of power. However, since you do not seem to play games very often, you may want to consider the slower HD 5570 instead which should be roughly equal to the HD 3850; probably a little slower. The HD 5570 consume roughly 20w of power from what I remember.
From:
http://www.xbitlabs.com/articles/graphics/display/axle-radeon-hd5670-1gb_3.html#sect0
Just beware that those maximum power consumption only occurs when playing games. Watching a movie will not stress the video card though so under normal conditions you are not going to see that 34w power saving with the HD 5670. (43w for the HD 5570). The HD 3850 uses about 21w for 2D/HD video conditions, while the HD 5670 uses about 16w for those same conditions.
Here is a PCI-e 2.0 HD 5670 for $63 after rebate:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102870&cm_re=hd_5670-_-14-102-870-_-Product
Unfortunately, the only PCI-e 2.0 HD 5570 card on Newegg is selling for $80. Therefore, it does not make sense to buy a less powerful card for more money. Perhaps you can find a similar card elsewhere.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814103177&cm_re=hd_5570-_-14-103-177-_-Product
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Clearly the best way to lower power consumption is to replace the power supply if it is inefficient. It is difficult to determine exactly how inefficient or efficient your PSU is unless you can find a professional review about it that goes into that type of detail.
Replacing the CPU with a dual core CPU will probably be the best choice, if you already have an efficient PSU, but my guess is that on average you may only save about 25w with mixed conditions. If you play games or encode video, then you will save more power.
The video card doesn't really seem to provide you with much power savings unless you play a lot of games. The difference between the HD 3850 and HD 5670 in 2D / HD video playback is around 6w.
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