computer woes

Hey guys.. i am having this problem.. everytime i boot up it is showing me that i have only 256mb of RAM.. i should have 768.. When i go into the bios it is showing 768 and also when i go into properties after right clicking my computer it is showing me that i have 768... I have done evrything from take out the DIMM cards and put them in one at a time to taking out the battery and its still doing the same thing.. please help??