No need to spend a small fortune on trying to use a notebook CPU... the Semprons are great choices. For the OP, what RAM do you need to recycle? What video card? What form factor is your case?
At the lower price range any of the AMD chips are great choices. If you have DDR2, then go socket AM2 (though they are almost all PCI-E). Socket AM2 Semprons start at $40 shipped for a retail box 2800+. Alternately you can get the low voltage 35W models, or just undervolt whichever chip you get. I did that with a socket 754 Sempron 2600+ with great results (your board has to support undervolting) on a Biostar Tforce6100 board, running the CPU at a few tenths of a volt below default, something like 1.15v thereabouts.
If you have DDR RAM, consider socket 939 because you can get a 3000+ Athlon 64 with guaranteed Cool and Quiet for $55 shipped, retail box. With pricing this low, would make for a better deal than a more expensive socket 754 Athlon 64.
For low power usage you have to take a holistic approach. What's the efficiency of your PSU? Look for the 80+ rating. What video card do you use? Many of the recent ATI cards use more power (creating more heat) than their Nvidia counterparts. How many HDDs are in the system? Each one can take up to 12W, so use fewer, larger drives. How many fans? If you are going for components that use less power, they put out less heat and you can get away with fewer or slower fans. Each one can take up to a few watts. If the motherboard you get supports it, set up Smart Fan to spin the fan slower if the extra cooling is not needed. Play around with Windows power settings.
Alternately you can do what I do. Manually turn off the HTPC if you aren't going to be using it. For instance, I only use mine in the evenings, so I turn it on the first time I go to use it and turn it off before I go to sleep. It's like turning off the lights - just make it a habit. Most lights don't turn themselves off, so why should the computer?
if you want to know how efficient a Core Duo is.
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