What does this mean 'usable' memory?

I have a NVIDIA GeForce 9800 GTX+ display driver too that has 512mb which I figure is important to mention as well.
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Kyle SantosQuality AssuranceAsked:
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Dave BaldwinFixer of ProblemsCommented:
It generally means that the operating system has reserved the rest of the memory for itself and possibly the video card.

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Kyle SantosQuality AssuranceAuthor Commented:

If you’re using a 32-bit version of Windows, the maximum amount of usable memory is usually about 3.12 gigabytes (GB).

So, 4gb of installed memory + 512mb of video card memory and its tapped out huh?
Dave BaldwinFixer of ProblemsCommented:
Actually, the hardware places the video memory inside the 4GB limit of a 32-bit OS.  While there are ways to use more memory, 32-bit Windows will only support 32-bit addresses which is the real limit.  Everything that is addressable must be included inside the 4GB space.

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Dave BaldwinFixer of ProblemsCommented:
By the way, most of this was setup for the Intel 386 which was the first PC CPU that could logically address 32-bit memory.  The hardware of the time could not do it and you couldn't buy that much RAM either.  The memory interface chips have always provided means to address different segment of memory although it used to leave big holes in the memory map.
Some of the Address space that would be needed to access your RAM is being used up by your mainboard's hardware. So that RAM is lost and can't be used. Generally you loose from between 512MB and 1.25GB that way, depending on your mainboard and it's hardware.

Besides that, many mainboards have built-in video GPU's that don't have dedicated RAM. Those share the main RAM of the PC with the video chip. On those boards you can often change the amount of RAM that can be used by the GPU within the BIOS. Your nVidia card probably doesn't share RAM with the mainboard, but that doesn't mean there isn't also an integrated graphic card in your mainboard which does share the RAM. So if your PC has such an integrated card it can help if you disable the integrated video from within the BIOS.
Kyle SantosQuality AssuranceAuthor Commented:
@rindi. Interesting.  I will check out in the BIOS to see if I can disable the integrated video.  

@Dave, Thanks for the information!  I will be back after I have checked this information out.
Kyle SantosQuality AssuranceAuthor Commented:
I checked the BIOS and there was no way to disable the integrated graphics on the motherboard (ASUS P5B).

Thanks for your help you guys! :)
Dave BaldwinFixer of ProblemsCommented:
Even on systems where you can install a plug-in video card that disables the motherboard video, the video ram for the plugin card must usually reside in the CPU/OS memory space.  I believe there may be some expensive video cards with a very large amount of video ram that is in the memory space of the GPU and only a small amount of ram in the CPU/OS memory space.
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