I am pretty sure the QM67 chipset of the E6420 can recognise 8GB RAM. But the OS will not address more than 4GB. Use a 64 bit OS is the only way of using more than 4GB of RAM. If hardware is not registering 8GB of RAM, then check if RAM is defective. Take out the old 4GB RAM and place the new 4GB RAM in that slot and see if laptop boot.