Usable Memory

Hey guys,

Sorry for being one of those "first post first question" members but I wouldnt have resorted to this if I didnt feel it necessary.

Anway, I have just finished building my new system which utilises the following hardwares:

Gigabyte P55A UD6
Intel I7 860
Kingston Hyper X 6G Memory
ASUS GTX 280 in Dual SLI Config
WD Caviar Black HDD 1TB
and other assorted things that arent neccessary to mention.

Put simply, I want to know why WINDOWS 7 is only recognising 2G of Usable memory even though it knows I have 6?

I have gone into the bios and have also realised that the bios only recognises 2G of usable memory but does recognise that there are another 2 sticks of ram available.

Does anyone know what the problem is or if this has been solved elsewhere?

You can presume that I have attempted all the basic troubleshooting prior to this post (Update BIOS, remove and switch around RAM modules, Reset CMOS, etc)

Thanks in advance,
Ed