So I am new at this ram issue, hope you can help

I have a GB GA-X58A-UD3R motherboard

I just purchased Patriot Viper 2 Sector 7 Ram 12GB kit, 4GB X 3 dims for a total of 12GB
I also have Patriot Viper Ram 6GB kit, 2GB X 3 dims for a total of 6 GB

I would like to install all 6 dims for a total of 18GBs if possible.

Both Dims are PC3 - 10666 @ 1333 Mhz

Both in Bios and Windows 7 64bit it shows 10GB usable and 8 missing. I have a dedicated graphics card so i know its not reserved.

How can i configure my bios and windows to use the whole 18GB if possible