The SPD tab of CPU-z is what you are reading. That information is programmed onto the eeprom on the ram module and cannot be changed without a specific program. Overclocking the ram does not change that info and never will.
PC2-6400 is currently the highest JEDEC spec for DDR2, thus this is the highest "table" that manufacturers will program into the SPD.
So the ram is going to say that its max bandwidth is PC2-6400, do you follow?