I'm currently trying to spec out parts for two computer builds and wanted to get some input on possible hardware choices. While the system requirements for each build is going to vary by a decent amount I'm hoping to keep power consumption down to a bare minimum.

The first computer is intended to be a server that will handle some light duty workloads such as FTP transfers and other web related activities. Since I'm planning to run the server 24/7 I'm trying to keep the power draw at under 20 watts under load or better. Since some of the workloads will most likely be write intensive it unfortunately rules out flash based systems such as the SheevaPlug. The D510MO looks promising but I'm not fully certain if it strikes the best balance between performance and power consumption for my needs, especially since there are some reviews that peg the power consumption at over 30 watts under full load. Granted those reviews didn't specify what other components they were using so those reviews could be outlier cases of people using inefficient builds..

The second system is intended to be a HTPC, I was originally intending to go with an atom processor for this machine, but the fact that I intend to do some transcoding and potential PVR work with this machine at times I don't think the Atom board would be the best choice overall in terms of performance. With this in mind. Since this machine is also most likely going to run 24/7 (especially if I turn it into a PVR) I want to keep the power draw low, but I'm willing to be a bit more flexible in terms of wattage in order to compensate for the increased level of performance. So far the best option I've found is a Zbox HD-ND22 CULV machine, but I have a sneaking suspicion that I could do better.

Recommendations on other necessary components such as PSUs would also be greatly appreciated.

I have a D510MO rig I've built for my kids to surf the net and play low-graphic-demanding games. This little pc is feeded by a Picopsu with a standard laptop power brick, has a 2.5" hdd, and a usb wifi adapter. Idles at 17w and go up to 21w under heavy load (playing games). Probably this kind of build is enough for a server too.

For an htpc I would do not recommend an atom board, save if it is a ION.

Dear OP,
You don't give enough information to determine your needs. But I'd say this:
A D510MO is probably overkill for your FTP/web server but it's cheap and easy to work with. Sure, you can build something that uses less power, but it would cost more money and/or be trouble unless you want to go for the single-core version which might not be a good tradeoff (an efficient power supply might save more power has no downsides other than price).
You might need a fairly powerful CPU to encode video unless you can use hardware encoding. Hardware-assisted encoding (with a dedicated card, a generic graphics card or some such) would likely use significantly less power but would be less flexible so I'd research that carefully if I wanted a low-power encoding system.

My needs are still in flux at this moment so I posted what I'm sure of.

From initial reports the Zbox HD-ND22 is essentially a souped up Atom computer, as it trades the Atom CPU for a CULV core 2 equivalent (the SU2300) but keeps the ION equivalent graphics card. Assuming I can get it stocked with enough ram and a decent enough hard drive I think it would serve as a pretty decent transcoding media server machine. And the fact that it draws just ~32W under full load is quite impressive.

EDIT: After going through the media library that I plan to stream most of the content will be SD files that will be played on an HDTV so there'll most likely be some transcoding going on to keep things looking right, and I do have a small but growing amount of HD content. Perhaps the Zbox may not be as up to the task as I thought initially.

It is good to know that the DS510MO is a good choice for my basic web server machine, means I have a bit of headroom for other tasks if I'm so inclined.

Consider using machines that don't have to be idle so much. Get ones that will operate with wake on LAN (for server, if usage is sporadic) or wake on timer (for PVR). i.e. turn it off rather than idling.

Why do you need two always on machines? (One web server and one DVR).
Could be more efficient to go for one server that handles all the always on needs (DVR, web server, media server, etc.)
By only having one machine idling you should save considerable power, so even if the combined machine uses more power at load it can still give a net power reduction. (e.g., Rather than web server idling at 10w, and a HTPC idling at 20w - total idle power 30w. If have one machine idling at 20w doing whole job, you save 10w x all the time it is idle, which would easily make up for a heavier use while the machine operating.)

If you can get by with one machine to do it all, then saves the embodied energy in second machine, which help offset the energy used by the more powerful machine.

Another option - if the DVR machine won't do wake on timer and you must have the FTP server always on, consider using your always on server to wake up the DVR when it is needed. (Give it a chron job to wake the DVR a bit before a program is to be recorded - using wake on LAN, e.g.)

For write intensive loads - rather than ruling out flash based systems, how about adding an external hard drive to such a system. (To handle the frequently changing material.) External laptop drive doesn't take a lot more power, and may be able to spin down when not in use.

Are there any tv tuners that incorporate hardware encoding into h.264 or at least mpeg-4? There seem to be some that do mpeg-2 which is not really much help.

On the topic of SheevaPlug like devices, are there any yet that incorporate a SATA controller? A cheap gigabit NAS with some server functionality is appealing but less so if you're getting bottlenecked by USB 2.0.

I also have one of the wd green 2 tb drives and am interested in building a system that could take several (say 4 max) of the same or similar drives when I want to upgrade later. In the specs they quote 6 W for read/write, but will the drives ever be drawing significantly more than that, say on startup or spinning up from idle? Also, is much of that power coming over the 5 V line at all?

We don't have devices that can measure and log transient power accurately enough to check startup power for HDDs, but every manufacturer cites much higher power demand for this. WD says 1.55A peak on the 12V line for 2TB Greens. That's 18.6W just on the 12V line. There's some power drawn on the 5V line, but it is much lower power than the 12V line. For example, on the WD Green 3TB, max measured current at seek on the 5V line was 0.45A, which translates to 1.8W -- compare this to 0.44A on the 12V line = 5.28W. In any case, 25W for max peak at spinup is not an unusual spec for drives. I would certainly make sure the PSU was rated to provide at least 20W per drive for a second or 2 anyway. The pertinent spec is peak power capability -- most PSUs can do higher than rated power for a brief period.