This should help you see what the guy above me was talking about :
Config A.
Memory running at 1000mhz multiplied by a bit width of 128 bits = 128,000 divided by 8 = 16,000 megabytes per second of memory bandwidth or 16 gigabytes per second.
Config B.
Memory running at 1000mhz multiplied by a bit width of 256 bits = 256,000 divided by 8 = 32,000 megabytes per second of memory bandwidth or 32 gigabytes per second.
In both instances the memory is running at the same effective speed but the example in Config B. gains twice the effective memory bandwidth over Config A. as it has twice the memory bit width
Here is the formula simplified :
( Effective Memory Speed ) x ( Bit width of memory bus ) = ( Total ) / 8 = ( Effective memory bandwidth in MBPS )
1000 x 256 = 256,000 / 8 = 32,000