The OS (Any 32bit OS) can only address 2^32 bits (4,294,967,295) of memory-mapped I/O (MMIO, henceforth). Damn near anything on your computer has an MMIO address; the BIOS, the buses, your VRAM (Which supersedes your DRAM addressing). The computer reserves blocks of MMIO addresses for VRAM, then buses/bridges/BIOS, then DRAM. Your RAM gets all the addressing space left, which is usually 2.5-3.12GB.

And that's that. You need a 64bit OS to get more.