That depends a little bit on the motherboard you are using. All motherboards have limits on the amount of ram you can use in them, and the configuration of the ram. Check your manual to see the sizes that are allowed. Many modern boards require ram to be in pairs when using more than one slot.
What is the make and model computer that you have? Or if this is home-built, tell us the make and model motherboard.
Edit: Drage beat me to the punch.