Also looking to the same .... but I guess you're looking for a windows solution (I am in the linux camp).
Now days for windows at least, NVIDIA & ATI, provide video acceleration in their onboard IGPs. So you don't need as powerful cpu, as you might think. In addition, the nova card processes the tv signal, so this is mostly a disk i/o process, not cpu.
You will need to think about power supply.. this is key... you need an efficient one... check out the Pico solutions (90% efficient). But you need to ensure that you calculate the wattage ... blu ray drive will eat the watts.
For a low power AMD solution think about 240e + AMD 785G motherboard. The X2 240e is due to be released at the end of this month (Oct 09)... so hang tight... the only down side is the on board hdmi... it can only handle 2 channel lpcm... so you can't have 8 channel output. (but no good for me as no video acceleration under linux). But if you can live with spdif or analogs then this is worth checking out.
The other option is using a socket 775 solution with the new celrons e3200/e3300. Again read up, and pair with a 9300/9400 IGP motherboard. The celrons are low powered, teamed with a good board you should be able to idle around 50Watts depending on cards, and hard disk requirements.
The intel onboard igp currently do not have the frame rate or video acceleration you need for smooth HD. So not many people go down this route for HTPC.
The ATOM 330 IONS could be of interest..... though might find they are a little slow for non-htpc duties. Plus normally they only have one pci, or pci-express slot (so after the Nova-t 500 card, no room for sat card, or other tv cards or sound cards). But you could get a ION solution idling around 25 to 40 watts.
There's quite a few low power disks around. For HTPC think green and think large (for your recordings). Maybe Western Digital 1TB low power/green. Better to have one or two large disks then lots of little power hungry little disks.
Hope that helps a little.
For more good information see
http://www.silentpcreview.com/forums/viewforum.php?f=28