Difference between 128 bit and 256 bit graphics card

Difference between 128 bit and 256 bit graphics card
the bit rate determines how fast the memory and the GPU can talk to each other basically, a higher bit interface, the faster they can talk.
256bit memory bus allows twice as much data be transferred at once, thus increasing performance over 128bit memory bus.
Just for an example: on a 128bit bus you could transfer up to 3GB/s, on 256bit bus you can transfer twice as much, which is 6GB/s.
128bits has exactly half the data width of 256bit. Meaning this: 128bit has half the memory bandwidth of 256bit. That's very important to know.
A 256-bit memory bus is definitely worth the extra bucks, 256-bit bus allows the card's RAM to flow freely or to read & write data to a much faster rate.