If you have 3 Ram Slots
try to put Two "1GB" Ram sticks on 2 slots and the 3rd one put "2GB" Ram stick
so 1GB+1GB+2GB=4GB
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If you have Two Pieces of "2GB" Ram sticks
then add the 1st "2GB" Ram stick on the 1st Slot
and the 2nd "2GB" on the 2nd slot..