Whats with my DDR2 RAM?

I have 2 corsair cm2x2048-6400c5 DDR2 RAM modules in duel channel on an xfx8200 mobo. PC6400 RAM is 800 MHz but the max bandwidth is marked at 400mhz in CPU-Z. Also why is there a 270 MHz clock and a 400 MHz clock? If I had a memory divider wouldn't it be in the bios? And is my FSB x 2 the equation for RAM bandwidth?