I recently swapped out 2 x 2gb for 2 x 4gb of of identical (other than size) Ripjaws 1600 on my MSI P55-GD65. The bios and CPU-Z both are showing 8gb and they passed two passes of memtest when I installed them, but today I had a few programs running with Civ 5 minimized and the pc was crawling. Fired up Task Manager and it showed 87% memory utilized and only reporting 4gb of RAM.
Run Msconfig by clicking start and typing in Msconfig in the search programs and files field. Click Boot then Advanced Options. Make sure Maximum Memory is not selected. If it is unselect it and reboot.
So it's working properly now. I tried swapping slots last night, checked the max mem setting, and ran Belarc Advisor. Swap didn't help, boot mem setting was fine, and belarc was reporting 4gb total but 4gb's in slot 1 and 3 (correct slots).
So I shut her down, swapped them back to the position they were in and low and behold, I boot into the bios and it's showing the 8gb. Weird, but at least it seems to be working properly now.