Energy efficient PC's - part one

I want to put together two PCS that are reasonably energy efficient. One of the most obvious ways to get an efficient PC is to buy a laptop, but I wanted something a bit more substantial. Im also comfortable maintaining PC's. I want a PC with reasonable performance (better than Pentium 4), One to two Gbytes of Ram, I want midrange 3D video cards,- good for most games. Lots of hard disk space, and not too expensive.
Ill be running Linux.
I started off looking at the CPU's. I have a preference for 64 bit CPU as they are more efficient than 32 bit CPU's under the correct conditions. On average you should process more data in each clock cycle. But then your programs have to run in 64 bits - this is where Linux comes in handy.
Remember the Transmeta CPU. It had up to 256kb wide CPU word and generated less that 10Watt heat at reasonable performance. No fans required so they didnt suck in any dust. Once a PC/laptop do not need a fan, they last very well - My son carried his Transmeta laptop around with him everywhere - even to school, and it is still going. Unfortunately Transmeta went bust as people werent that interested in efficiency at that time. Another option is to buy a VIA Epia TC6000E Motherboard with CPU, but they were a bit low in performance for my liking.

Anyway back to the topic. CPU. Energy usage of AMD chips are freely available. The sites all quote TDP (Thermal design power), which is the maximum energy usage (heat). So the assumption is that you can do better and use less power if the CPU is in an energy savings mode. Im happy to work with an upper limit. Im not sure what to assume around the hype of energy efficient modes, but an upper limit tells you the Watts stop here. For example, I am skeptical about statements like, if you have a dual core, and you switch one core off, you use half the power.
You can select the energy usage on the AMD site to see the CPU's with the selected power ratings. There is a lot of hype on the Intel site, but the technical details are hidden away. The following 35Watt/45Watt (TDP) AMD Athlon 64 CPUS would suffice: (3500+ Orleans, 3800+ Windsor, 3800+ Lima) - Socket AM2 Motherboards.
I may still consider Intel , see Scott Mueller's forum for some thoughts on this. In the Intel CPU range the 65Watt (TDP) Intel Core 2 Duo Processor is the most viable option. Apparantly the Intel Duo is a merger of the Intel CPU tecnologies from the desktop and laptop areas. The upside of the Intel Core 2 Duo Processor is the performance, the downside is power usage and a substantial price difference.
Some more ...