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#21
A 32 bit system only has a 4GB address space ( 232 ). The video RAM has to fit into that space and accounts for the 'hardware reserved' 1.1GB in my x86 screenshot. This means that 1.1GB of the installed RAM cannot be addressed, those addresses are being used for accessing the video memory.
A 64 bit system has a 16 exabyte address space ( 264 ). With 4GB installed RAM the OS can map the video ram to an address higher than 4GB meaning that virtually all of the installed RAM is available for use.