bambam29 :
what is "matching set" sir?. you mean i need two 8G to achieve dual channel?.
They should be bought as a set. Such as 2x8GB packaged as a pair for guaranteed dual channel.
If you buy the exact same make and model they may not achieve dual channel. As there may be slight differences. Since they may have been produced at different factories. Usually you'll get dual channel buying them separately. Sometimes you won't.
Your lowest chances are if they are from different manufacturers but the same specs.
Different sizes of memory have no chance of dual channel operation. As they must be the same size. They'll operate in single channel mode.