I just finished building a new HTPC so i thought I'd share some pics and info.
ASRock E350M1/USB3 mini-ITX M/B - $159
Aywun MW-107 Mini-ITX Case Black - $59 (I had planned for an Antec ISK300-65 but couldn't find stock)
Kingston 2GB DDR3-1333MHZ - $28
Kingston 30GB SSD V-Series - $79
Total - $325
Existing components were - Windows 7 Home Premium 64 and a Logitech Dinovo mini edge.
My original plan was to create a dedicated XBMC box with XBMC live. However I soon discovered that ATI and linux don't play nice. So W7 was my backup.
In the end its a solid system that I'm very happy with. The goal was a low cost, low power streamer that can playback all my rips perfectly with a great interface. Some bonuses were things like access to web content like ESPN3 (you could go even further down this path with a U.S proxy).
Now some pics (sorry about the quality)