You really just don't get this do you?
DDR, DDR2, DDR3, GDDR2, GDDR3, & GDDR4 all at 800MHz will all have the same max theoretical bandwidth. They all send out data twice per clock. GDDR5 sends out data 4 times per clock.
256bit DDR2 @ 800Mhz is the same speed as 128-bit GDDR5 @ 800MHz.