I recently bought two new 2 GB RAMs for my MacBook White (2007). I tried replacing the RAMs that are already in the MacBook (two 1 GBs) but it doesn't seem to work. I could not boot my computer. However, if I just replace one of them and leave the other, it works. I currently have 3 GBs of RAM. So how do I get both of the RAMs to work in my MacBook?