Acer Aspire Revo. You can pick one up for $200 at Best Buy in store or buy online at Newegg, Amazon, etc.
Specs: Atom 230, nVidia ION LE, 1GB RAM (expandable to 2 or 3), 160GB HD, HDMI & VGA, 6 USB 2.0, 1 eSATA, 1 Gigabit ethernet port, SD slot. Pulls 65 watts max and physical size is 7.1"x7.1"x1.2".
This can play back full 1080p without dropping frames and will run XBMC or Boxee under both Windows and Linux.
I purchased one yesterday and so far I've run XBMC under Linux and Boxee under both Windows and Linux and will be sticking with Boxee running under Ubuntu 9.10. The performance is great and currently the only down side is that flash playback isn't very good but Flash 10.1 will fix that.