This is today's 640KB limit. A 32-bit processor uses one 32 bit word for each pointer. This limits memory size to 4GB (2^32=4G). As you have 4GB + your graphics memory, you cannot see all your memory.
I think that more recent ones do not write off the top GB to make space for the video ram, but I am not sure. It may be that you do have access to 4GB total space, but it is not displayed correctly.
You cannot see all your memory unless you switch to amd64.
Edit: I did, of course, forget about PAE, which is discussed below. Think about it as the 'himem.sys' of the 4GB limit. That's if you don't have to use it. If you have to use it, try not to think too much about it. It's better that way.
The only dumb question is a question not asked.
The only dumb answer is an answer not given.
Last edited by robbak; 6th April 2009 at 12:21 PM.