The physical address space of the IA32 architecture is 4Gb, but the upper 1Gb is usually reserved for memory-mapped I/O, leaving 3Gb of RAM available. Above 3Gb you'll need PAE as alluded to above. Video memory is not directly addressable to the CPU, only to the GPU. The graphics card maps an aperture into the upper 1Gb; that aperture may be the size of the entire VRAM if there is sufficient space, or only a portion of it if there is large amounts of VRAM. This is partially correct. With PAE enabled you will have access to the entire 4Gb of RAM, the 512Mb of VRAM doesn't affect anything as it's probably going to be mapped somewhere in the now-64Gb physical address space.