Installed Memory vs RAM Usable by Windows
 

Most PC users are familiar with the 4GB system memory limitation of 32-bit Windows Operating Systems, which is why 64-bit computing has become the standard for computer enthusiasts. Occasionally though, a computer system will not report the correct amount of RAM installed. In this article, our 64-bit Windows 7 test system had 6.00 GB of installed memory but indicates only 4.00 GB usable RAM available. While some may dismiss this as a case of defective RAM, you might be surprised by the culprit.

http://benchmarkreviews.com/index.ph...sk=view&id=541

This happens to me too from time to time. What you need to do in most cases is reseat the CPU, not replace it.

Agreed. Damaged socket, mis-aligned CPU, or even overly aggressive RAM settings. I can't imagine the CPU controller being "bad" unless a memory controller was degraded through overclocking?

had this happen too; culprit turned out to be the motherboard