What does this mean 'usable' memory?

Ah.

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?

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.

https://msdn.microsoft.com/en-us/library/windows/desktop/aa366912%28v=vs.85%29.aspx

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.

@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.

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! :)

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.