Only one stick of RAM works.

So I recently purchased two 2GB stick of RAM to update my Mac (Late 2009, White Unibody). When I replaced both sticks, I get either a blank screen, or the computer beeps three times to me, which lets me know the RAM is faulty. But when I take one of the sticks out, it starts to work. I've switched and this works with both sticks of RAM in the upper slot. The lower slot wont use these new sticks, but it will work with the 1GB sticks that were originally in when I purchased the Macbook. Any ideas?