I ran into the exact same problem on my Vista 32-bit box… the memory showed up properly in the BIOS, but not in System Information. It turns out that Microsoft recently released a KB article to describe the problem (although they didn’t go into the level of depth that you did… great job!)
http://support.microsoft.com/kb/929605/en-us
The system memory that is reported in the System Information dialog box in Windows Vista is less than you expect if 4 GB of RAM is installed