You can get a program called CPU-Z that will tell you a bunch of information about your computer and exactly what you have installed for your ram modules. It does seem that your computer is recognizing them as a 256 stick and a 128 stick which is a possibility, but we will see.

Download CPU-Z from the attachments below this post (or search google for it) and unzip it and double click on the cpu-z icon inside the new folder you created. Then click over the top tabs to the one called SPD (I have a picture attached to show this.) In that tab there is a drop down menu in the top left side for the different slots that ram can go into in your computer (either 2, 3, or 4). Two of them should have information about the ram that is installed including the manufacturer, and the ram size.

If they both say that they are 256 then report back here as we can see further what to do. However, if one is a 128, and the other is a 256 then you know that this is what they really are. If you really think that you installed 2 256 sticks you can open the computer case and examen the sticks as they should have information pertaining to the size that they are.

Hope this helps