I'd personally just go for a D510 (Pineview) out of the Atom range. Dual core, HT, integrated graphics, everything all ready to go.
Totally agree. 70 euro for the new Intel D510MO (Its Pinetrail not Pineview BTW) is out of the box the solution you are looking for. I built one for fun a couple of weeks ago. All I added was a spare 2.5 inch laptop drive, a pico passive PSU, 2GB ddr2 and a case (t350).
It boots windows 7 about as fast as my overclocked i7860 (at 4ghz) and for surfing the web etc it is as fast.
Its passive, silent and uses 13watts of power. Couldnt recommend it highly enough.
Disadvantage is that its VGA only not dvi or HDMI.
Also it only has 2xsata. I was hoping for 4 as then it would have been the perfect Windows home server board. Then again, it has a pci slot for an extra sata card.
Also a sneaky point, that not a lot know about yet, is that it can take the new broadcom crystal HD mini pcie card. My friend and I tested this with it last week. (the crystal HD is 20 quid of ebay). Using the pinetrail, the broadcom card and the beta opengl build of xbmc for windows it plays 1080p flawlessly. Pretty awesome for a totally passive 13w HTPC.