From the original 4gb (4x 1gb) DDR2-PC-5300 (667 Mhz), we upgraded it to 8gb (4x 2gb) DDR2-PC6400 800Mhz but when i boot the Server it doesn't go to POST. What seems to be the problem? X7SB4 is capable of up to 8gb.
You need to use dual channel memory and must be populated in pairs.