I have 8 GiB of physical memory, but only 6.56 GiB of it is available. See the screenshot below:
It is not shared with an onboard video card - I have a NVidia NVS 3100M video card with 512 MiB of its own RAM. It is also not limited by 32-bit, obviously, or it would be 4 GiB.
In a previous laptop I was missing a bunch of RAM due to 64-bit issues and I thought that it was that field that showed the difference, but as people pointed out that's just the actually free RAM. That's what I get for not checking resmon.exe :P