1600 Mhz RAM to fit in for 800 Mhz BIOSTAR G41D3C MotherBoard

Solution
Although the Samsung memory appear to be identical, there is likely to be variation in characteristics between the two modules (existing module and the module on eBay). Because of this variation in characteristics, the two modules may not work together (properly) in dual channel mode. However, if lucky, the module characteristics may be identical and the modules might work together. But this is not guaranteed.

It is recommended that a dual channel kit be purchased instead to ensure memory module compatibility.

Note, by default, the modules should be run automatically at 1066 MHz by the system to ensure system compatibility (all else being equal).
Although the Samsung memory appear to be identical, there is likely to be variation in characteristics between the two modules (existing module and the module on eBay). Because of this variation in characteristics, the two modules may not work together (properly) in dual channel mode. However, if lucky, the module characteristics may be identical and the modules might work together. But this is not guaranteed.

It is recommended that a dual channel kit be purchased instead to ensure memory module compatibility.

Note, by default, the modules should be run automatically at 1066 MHz by the system to ensure system compatibility (all else being equal).
 
Solution