You can put higher speed memory in a computer spec'd for a lower speed, but you DDR3/DDR2/DDR1, etc are not interchangeable. And to ensure no one makes that mistake, they all have differently place "notches" so physically you cannot do so.
There are a very few desktop motherboards out there that have both slots (and those are fairly old now as they came out during the transition of DDR2 to DDR3), but for the most part it's not possible for PC's or Macs