I have this exact problem fixed ! With Gigabyte p55a ud3r and Kingston KHX1600C9D3K4/8GX.
When you buy ram, you get them in pairs, as it turned out each pair/memory stick can only work on memory channel 1 or memory channel 2.
If you put a pair of memory that designed to work only on channel 1, on channel 2 (like I did unknowingly). The Bios only recognizes them, "BUT" not made available to use.
Windows 7 can only see the total ram installed (in my case 8GB), but can't make use of the other 4GB on memory channel 2, and
Windows 7 thinks the 4GB memory are "HARDWARE RESERVED".
So I advices you to switch them up between the memory slots, like I did, and now I only have 5 MB HARDWARE RESERVED instead of 4GB like before when I put the ram sticks on the wrongs slots/channel.
And now, have a joyful day !