Windows x64 but only reading 4gb memory

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hi i have 6gb of memory installed on a core i7 system and it shows that i have 6gb installed but says only 4gb useable. now i checked to make sure it was x64 OS and indeed it is how do i get it to read the other 2gb? i have 3x 2gb sticks in

 

[WINTERLORD]

EX58-DS4 gigabyte socket 1366 board no intergrated graphics