I recently bought PC with 4 GB RAM installed. Windows Vista recognizes 4GB (as shown in Control Panel:System) but I think it uses only 3GB (as shown in Windows Task Manager). What gives?
When I upgraded to Windows 7, same problem appeared. There is a workaround. Download kernel patch, apply and see it for yourself (by inspecting total memory recognized by Windows 7)
Linux (no problemo)
MemTotal: 4117664 kB
MemFree: 2866824 kB
Linux surya-desktop 2.6.28-11-server #42-Ubuntu SMP Fri Apr 17 02:48:10 UTC 2009 i686 GNU/Linux