If you want standby for a long time, you will need a very efficient board. That means going with an embedded processor via board. Out of those, I reccommend the relatively new CN series: http://www.via.com.tw/en/products/ma...erboard_id=400
Total average power consumption is 16W. And with the use of DDR2 memory and a special chip to cut memory power usage by a third (good for standby).
So that takes care of CPU and Mobo. Then the M2-ATX is cheap and can power it. Then all the rest you need is the HD and RAM.