(correct me if i'm wrong somebody who knows) but did you enable the 'memory remap feature' in your bios? I was just reading about overclocking here -http://www.tomshardware.com/forum/240001-29-howto-overclock-quads-duals-guide
The writer mentioned this feature in his BIOS runthrough. Apparently it will allow your mobo/OS to utilize all of your RAM
The only other setting worth mentioning here is the so-called “memory remap feature.” If you are running with more then 3 gigs of memory, and you want to actually have the BIOS/OS see it, you’ll need to enable this. Also enable this if you’re running a 64-bit operating system.