Question My hardware reserves 5,3GB of memory out of 8GB

[LahaRyms]

Ive recently got a HP Compaq DX7400 with C2Q Q6600, 2GB of DDR2 800Mhz and MS-7352 Motherboard. Ive put inside my old GT740 GPU and wanted to upgrade the memory to maximum supported by the MoBo which is 8GB DDR2 800Mhz but after installing the memory it appears that only 2.7GB are usable and Im thinking it is BIOS related issue, also tried cleaning the slots, reseating, removing 2 sticks and it always shows that 2.7GB only is usable, can someone help me figure it out please?

 

[rgd1101]

what the version of windows? 32bit?