"Low-power dual..." is a oxymoron.
Unfortunately, your best option for dual processors at a reasonably low power is hard to find in 1U form, at least pre-built.
The "consumer"-oriented AMD Athlon X2 chips (Socket 939 and now AM2) are a better proposition power-wise than the Opterons. The Opterons, on the other hand, are better-suited for servers. It's actually difficult to find a single-chip (which is all you need for dual CPU nowadays) Opteron server, though. There are some special lower-power Opterons, but them come at quite a premium, and also take a performance hit.
Even if you want to build your own, server-oriented Socket 939 boards (i.e. with built-in video, optionally built-in SCSI) are few and far between.
If you REALLY want low power, there are a small handfull of boards available that take either Intel or AMD chips that are meant for mobile applications.
If you don't need a speed demon, there are some additional options using alternative chips. (Non-AMD, Non-Intel.) But not dual. Surprisingly, these ARE fairly readily-available in pre-packaged form. Do a search for "short-depth server". There seems to be a niche for these short-depth (typically 14") 1U low-power servers. They can be packed into both the front AND back of a typical enclosed cabinet due to the short depth.