Your MBP will have two chip slots that are stacked on top of each other. When you remove the top chip, you'll see the second chip sitting below it. It is best practice to replace both chips with the same amount of memory. So your current computer should have 2 - 2gb chips, so if you want 8gb total, you would install 2 - 4gb chips. Or for 16gb total, 2 - 8 gb chips.