Windows 7 can only use 7.87 GB of my 8 GB RAM. Why is this? I am using a dedicated graphics card, so I ruled out the use of shared memory with my built-in graphics.

Windows 7 can only use 7.87 GB of my 8 GB RAM. Why is this? I am using a dedicated graphics card, so I ruled out the use of shared memory with my built-in graphics.

This happens when the integrated GPU of your CPU is still enabled. So go to the BIOS/UEFI and disable it if you only want to use the dedicated one.
Answer from Microsoft Website: http://support.microsoft.com/kb/978610
Quoted:
The amount of usable memory in the examples are not exact amounts. Usable memory is a calculated amount of the total physical memory minus "hardware reserved" memory.
They have more information there as what may caused it, and what to try to test the RAM.