The ram slots are completely different and you cannot put the DDR2 stick in a DDR3 ram slot. When you look at the bottom of the DDR2 ram stick you will notice a space and that lines up with the ram slot and that space is different in the DDR3 stick and slot so that is why you can't use them together. Besides that they are different technology wise as well , a motherboard will either support DDR2 or DDR3 and not both.