memory register has allot to do with memory bandwidth.........
a 256 bit card has twice the memory bandwidth of a 128 bit, so umm
128 bit at 1000 mhz = = = = == = 256 bit at 500mhz memory speeds
..............
edit ---> lol i should read the rest of the posts, not just the one preceeding mine forget everything i said i was talking gddr