I am in the middle of putting a system together (just waiting delivery of the cpu). I bought the 2 x 2gb crucial (CT2KIT25664BA1339) for dual channel operation.
I have just this minute been given (birthday present) CT51264BA1339 a single 4gb piece of ram
So now i have 2 x 2gb and 1 x 4gb. I have the Asus P5P43TD which is dual channel with 4 slots. I have the 2gb rams in the dual channel slots. Will fitting the single 4gb piece in one of the other spare slots lose me the dual channel status of my other memory.
Yes, adding a 3rd module to a dual channel machine would disable the dual channel across the board. That being said, you should have better performance running 8GB in single channel than you would running 4GB in dual channel.