Well I have my low power[ish] PC put together now, I had a couple of issues while assembling it.
1 - The Foxconn G31MG-S isn't available anywhere, it looks as if it won't become available for a few weeks yet. Since I want the computer now I have used a boring/generic/cheap Foxconn motherboard instead. As a result consumption is rather higher than predicted but is still quite acceptable at around 50 watts when idling with two disk drives and a CD/DVD drive. I could save a watt or two by unplugging the CD/DVD but it's hardly worth the effort.
2 - Some power supplies don't handle SATA drives well. I wasted a load of time on this. I ordered a case *and* a power supply when I ordered the parts for the system. The case (to my surprise) came with a 355 watt power supply so I though I might as well use that and keep the ordered (430 watt) one as a spare. I built the system and it worked OK until I added a second SATA drive, or at least I saw the same symptom a few times with only one SATA drive but put it down to misconfiguration. However when I tried to get it going with two drives it always just powered up for two or three seconds and then powered down again. A bit of searching suggested that it *might* be a power supply problem and so it was, using the 430 watt supply instead has made it work perfectly. Amazing that a system which idles at 50 watts and always uses less than 100 watts can't start up with a 355 watt power supply. Apparently SATA drives hit the 12 volt rail harder than IDE drives used to.
I'll probably buy a Foxconn G31MG-S when they become available, more for the fun of seeing how low power I can be than for any real saving. The difference of 20 watts will only save about 0.5KWh per day so the annual saving is only around £20.