I cant found where write about it in your link =/

The Inspiron 1520 laptop can be upgraded to 4GB of Ram, but it only has two memory slots.  This means that you will need to have 2 Ram modules of 2GB each.

If your system came with 2GB of ram, there is a good chance that it has 2 1GB memory moduels.  This means that to get 4, you will need to replace both of the modules.  If, however, the system came with 1 2GB module installed, you will only need to purchase one.

To see how your memory was configured on your system, go to http://support.dell.com/support/topics/global.aspx/support/my_systems_info/details?c=us&cs=19&l=en&s=dhs&~tab=2&~ck=anavml Enter your service tag and you will be able to see if there are 1 or 2 memory modules in your system.