Help. this may be covered in a previous topic, but i can't find a simple answer. i have acquired a dell optiplex 780 with 4 memory slots that came with 2 x 1gb modules in 2 slots. i want to upgrade to at least 4gb, but am confused over what i need to install. the os is windows 7 home premium 64 bit. i've tried 2 different 1 x 4gb module and i get lights 3 & 4 showing and the computer won't start windows. do i need to install a pair of matched modules rather than a single one? if so, does it matter which slots they go in?