Ubuntu detects only 3.2GB RAM on a 64bit OS [duplicate]

Your RAM modules are compatible with the board but the memory frequency is what the limiting factor is - and that is why you only have 3.2GB available to the operating system, 800MB is reserved for the motherboard system graphics.

Your intel board only supports 4 GB maximum total system memory using DDR2 800 DIMMs according to the product spec page from intel

The product numbers of both brands of RAM modules NT2GT64U8HD0BY-AD and OCZ2N800SR1G listed in your lshw output indicate they are PC2-6400 modules at 800MHz.

It appears from your screenshot that your bios is limiting the memory frequency to 667MHZ and does show them being 800MHz modules.

The reason you can see 6GB of RAM installed via lshw is because Ubuntu is listing all the hardware installed on your system - not what is in use.

To allow your board to use more RAM you need to purchase modules rated at 533 or 667MHZ - then you will be able to use up to 8GB.