What are they and why are they better than single?JimCYWG

It's only a technical detail. Most 512MB sticks are double-sided. If you'ld get single-sided modules, your FlyTendo would only be using one bank per RAM-channel and likely only be using half of the potential FSB speed. To make it even more confusing, although double-sided modules, often, the individual RAM-dimms are only located on one side of the RAM-modules. Hence, just looking at the module itself will not allow to identify which sort it is. For most modern systems it's advisable to use one pair of double-sided modules. Hope this helps, good luck and kind regards Jaap