A few motherboards (very rare, none of them new) support both DDR2 and DDR3 but only one type at a time. I will assume your machine doesn't have such a board if it is a prebuilt computer. without dual support a motherboard is locked to one type of memory permanently. The pinout of a DDR2 module and a DDR3 module is logically different and the notches in the pins are different to prevent users from installing the wrong type of memory in the wrong slot. If you did manage to mismatch a module and slot it would not work and probably overheat until it started smoking or caught fire.