Zareh,
it is possible to use more than 6GB of DRAM operating at 1600MHz, however it is not fully supported by Intel specifications as it exceeds the qualified design of 1333MHz
Check out corsair CMP24GX3M6A1600C9 and kingston KHX1600C9D3K6/24GX these kits are made specifically for 24gig setups that allow you to overclock the memory up to 1600mhz
24GB of memory at 1600MHz is not guaranteed to work in all system configurations but the two above kits have been known to work when overclocked to 1600MHz