The new memory must be a type the computer can use.
There are different types of memory, high density boards, low density chips,
High density chips, low density boards, etc... Get the correct memory for that machine.
My understanding, this is what I had.
Do you think not?
I'm not sure what you have.
The G Skill F2 6400CL5D-4GBNT PC26400 are not 2GB in a pair, they are 4GB in a pair. You said you added 2GB by adding a pair of these. I think you added 4GB with them.
This normally would not be a problem but that board only supports 2GB of memory. See specsHERE
If you download and install a new BIOS, you may get 4GB detected. Asus now describes the board as capable of 4GB, but we don't see when that change took place. You can download a new BIOS from ASUS HERE.