He hasn't reinstalled since he swapped out motherboards. I would be much more concerned about that than your blanket statement of a CPU not using the RAM enough.

The CPU doesn't use RAM, the OS does. More memory means you can cache more information for the system to use, generally speeding up a system because if it's in the system memory, it doesn't have to hit the hard drive at all. The speed doesn't matter anyway because the memory will just clock itself down, not causing a bottleneck, unless SPD is turned on in the BIOS.

Defragging will likely do nothing as well. Reinstall the OS.