|It would help to post the make/model of the motherboard. Only 3 slots is a bit odd for a board that supports a Core 2, usually it's 2 or 4 slots.|
Take the memory sticks out & snap them back into place again. Make sure they're both fully seated in their slots. You should hear a 'click' on each end when they lock into place. Also, if you're using onboard video instead of an actual video card, some of the memory will be used for onboard. How much depends on how you have it configured in the BIOS.