I found someone on another forums that appears to have a similar issue and fixed via the method below. Take a look and see if that helps at all.
Just wanted to help out others that end up with this problem seeing as it took me forever to figure out a fix (with no little help from researching via google).
Anyway im running the following rig:
Intel Core i5-750 Lynnfield 2.66GHz (w/ Cooler Master Hyper 212 Plus) - OC'd at 4.0Ghz
8GB (2GB x 4) G.SKILL Ripjaws Series 240-Pin DDR3 SDRAM
SAPPHIRE Vapor-X Radeon HD 5850 (Cypress Pro) 1GB 256-bit GDDR5
ASUS P7P55D Deluxe LGA 1156 Intel P55 ATX Intel Motherboard
CORSAIR CMPSU-750TX 750W
Intel X25-M Mainstream SSDSA2MH080G2R5 2.5" 80GB SATA II MLC (SSD)
Western Digital Caviar Black 1TB 7200 RPM
Antec Nine Hundred Two Black Steel ATX Mid Tower
Windows 7 Ultimate 64 bit
Any way i had the problem when i installed 4GB more of memory (to make 8GB total) that my computer on recognized 4GB as usable and put the rest under "Hardware Reserved".
Steps i took for fix:
In bios make sure "Memory remapping" is enabled
Update bios to latest version
Voltage for DRAM to company specs (1.60v in my case)
---at this point none of it fixed the problem---
but then i RESEATED THE CPU AND DIMMS - afterwards my bios detected the Reseat cpu as a new cpu and subsequently detected all 8GB of memory.
So in my case if you have a rig similar to mine you have to reseat your CPU and heatsink for your 8GB of ram to become usable. Don't waste your time looking for an easy fix - it took me hours to no avail. Reseating takes some time if you do it right - but in the long run your PC runs cooler and it solved this problem.
Hope this is a lifesaver for others that have this problem!