Op mentions that the mobo is 945 specs - and running off the onbaoard G945 chip, correct? if it is the case, than some of your memory is being shared to help run media/entertainment applications.
from this site
you can have a max of 4GB of ram. The catch here is that you can populate the maximum using all 4 dimms, but if you had a later mobo say an x38/48 chipset mobo than you could've used only 2x2GB sticks to saturate the 4GB max limit.
* welcome to the forums newcomer!