Lack of time and upcoming move made me put this great system on eBay.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=5125054969
Tons of detailed pictures inside the listing, but here are the main specs:
Celeron 300, 64Mb, 20Gb, CD/DVD bay, 2 PCI, 2 USB, Ethernet, VGA, a bunch of AV inputs and outputs, remotem etc.
Made by Fujitsu-Siemens, Activy 300 was meant to become a set-top platform for customized applications. Big in Europe, these units only started showing up in US. I was lucky to get a 'virgin' brand-new unit on eBay couple months ago, but due to extensive travel schedule I had only half a day to tinker with it. Unfortunately, I am hitting the road again for an extended period of time and physically will not be able to dedicate enough time to making this computer a home theater PC. Maybe you can?
The system comes with a Celeron 300 Mhz CPU and 64Mb of RAM, but both should be upgradable. As you can see in the pictures below, it has a custom motherboard with 2 PCI slots on the riser card, two USB ports on the back, Ethernet, VGA and numerous AV in-out ports. The motherboard also has a Real Magic hardware DVD de-coder chip. This system is totally virgin - its Seagate Barracuda IV 20Gb hard drive is totally blank. Other features include a bay for a CD/DVD drive, remote, additional IR receiver for remote, a slot for smart-card reader, a space for an LCD dislpay.
The system was made incredibly quiet - even with the cover off I had to listen closely to determine if it was actually running.
There is some end-user documentation included, but it is pretty basic. I even had to experiment to find my way into BIOS.
The system would be a great project to work on!