yeah, need to know more of your goals as well. Depending on the type of workloads, some processors are more efficient than others. For example, if your computer is going to be idle 70% of the time with light workloads, an Atom N270 would be very efficient. If there will be significant work loads and or time/speed dependent tasks, Some Athlon II "e" processors and Core 2 Duos would be most efficient. For overall performance and efficiency the core i5 is a good choice. Idle power usage is better than the i7 920, performance comes close in some benchmarks, and uses less power at load.
Thanks for the answers.
My goal is to reach the 100 to 200w maximun power consumption.
The system is going to be used for bluray playback, dvd scaled to 1080p playback, hdmi output, internet browsing, hd streaming video (youtube, vuze, etc), some games on ps2 emulation software scaled to 1080p, and some office work. Most of the time it will be browsing the net, and 1080p scaled video content playback. However, the energy consumption is my main concern.
Budget: $500-750 (without the monitor)
Another concern is the possibility of bypassing the bluray HD sound to an AV receiver capable of such decoding without any frequency or sampling cut.
I've been reading about the Athlon II "e" series, but have no idea about the motherboard and video (on-board or not).
The core i5, core 2 duos, are big question marks...
There's another interesting option coming soon if you can delay your build until early next year. Do some searches on the core i3's intel is planning, and you'll probably like what you see. They will basically obsolete all current generation dual core processors since their idle power, and performance per watt ratios are going to be very good. They will also feature an on die GPU, so these systems will have benefits of dedicated GPU processing, with power consumption that more closely resembles an integrated chipset.
If you can't wait for the next generation, the ATI graphics integrated chipsets are the best low power performers currently. They will easily handle 1080p content, blu-ray, etc. They will not do so well with 3D gaming, but no integrated graphics solution will.
Other tips to reduce power consumption: go with low voltage DDR3 RAM when possible. 2.5" laptop drives consume easily less than a third of 3.5" drives. When choosing a PSU, get the optimal size which should be max capacity of twice the system's peak load. Look for PSUs that have a gold 90+ rating, not many out there and expensive, but worth it in terms of energy efficiency. Use a good CPU cooler, the cooler you can keep the system, the less energy you will expend on cooling it down.
Lastly, if you really want to make the most out of power savings, research undervolting. It's possible to keep stock performance while reducing power consumption by achieving a stable undervolt. This doesn't make for huge power savings as advanced power management is very good these days, you can still shave off a few more Watts.