can i put 2 different types of ram in my computer?
You are right about mixing different RAM modules — if there's one thing you absolutely can't mix, it's DDR with DDR2, or DDR2 with DDR3 and so on (they won't even fit in the same slots). RAM is pretty complicated, but there are a few things you can mix and a few things you shouldn't. In any case, I don't recommend it.