My desktop not detecting the actual RAM
 

I had 1gb RAM(512x2 DDR2). I upgraded it by 2gb (DDR2). When i put the new RAM card in one of the four slots it detects the right amount i.e 2gb. BUT when i put the old one 512 or 512x2 along with the new one 2gb it detects only 512 or 1gb. Please help!! Motherboard Specs...