--> many motherboards like "single-sided-RAM"
if you want to upgrade your old pc with 2 RAM-Slots with max-size of 2 GB RAM or 4 GB or so and one slot is used (perhaps with a doublesided RAM module)
--> take a single-sided module for your second RAM-Slot otherwise it may happen that the second module will not detected. in this case the settings in the post above will not help!
(go to your store and exchange the wrong RAM-Modul)
+ with maximum-sized RAM placement not all kind of possible modules will detected and recognized.