Well, this seems to be a common problem. Before answering please read well this thread.
I Have Windows 7 Professional x64 with 8GB installed (2x4GB DDR3 Kingston). I installed the windows with the 8GB from the start. Windows recognizes 8GB installed but only 3.2GB usable.
In the Resource Monitor 4.9GB are "Hardware Reserved". I have a nVidia GTS450 graphic card, but the nvidia control Panel "System Information" shows that it uses 1GB from the card framebuffer and 1.4 GB shared system memory.
The M/B supports 8GB
I didn't find any memory remap option on the BIOS, but it clearly detects 8GB (the BIOS).
I tried checking (and unchecking) the "Maximum memory" option in the msconfig > Advanced Options. When i chek it it automatically appears "8192" (8GB?), but when i reboot and check again it appears "0" (which means auto??). Nothing happens at all.
Further PC specifications:
MSI H55M-E21 M/B (socket 1156)
2x4GB DDR3 Kingston (PC3 - 10600 CL9 240 - Pin DIMM)
See the images below
P.S: please dont tell me to check if my OS is 64 bit. It is