I need to upgrade the RAM in my HP ZV6000 notebook to load Windows 7 from the current 512mb. This is configured with (2x) 256 mb cards, one in internal and one in external ports. Do I have to match the internal size and external size when I buy new memory? I would just like to buy a single 1gb card and replace the 256 mb card in the external port, which is the easiest to access. Would this work OK?