This is the limitation of IA-32 architecture and 32-bit OS - the total addressable memory is 4GB, and certain memory range below 4GB is reserved for BIOS, PCI-E device, VGA, etc. See http://www.intel.com/support/motherboards/server/sb/CS-010458.htm.
64-bit operating system can use memory above 4GB. If you have to use 32-but OS, try the following BIOS setting to get more usable memory:
Advanced => PCI Configuration => Memory Mapped I/O Start Address => 3.5GB
Note: If any device is not working properly after you choose 3.5GB, try 3.25GB, 3.0GB and 2.75GB in order until all devices are working properly.