bandwidth

is the product of the interface and the data rate. If your interface is twice as wide, you get twice the memory bandwidth between the GPU and the graphics memory. Just to keep up wityh the fill rate, each pixel needs 4 bytes (32 bits) for every frame meaning that you need 1600 x 1200 x 4 bytes for every frame at 1600 x 1200 resolution. Make that 200 fps and you need 1.5 GB/sec bandwidth. Add 4 x AA and you need a minimum of 6 GB/sec.

Take a 256 bit interface, you need 192 MHz memory speed, with a 128 bit interface you need 384 MHz and so on.

Keep in mind that this is only the read traffic at the theoretically maximum bandwidth and does not take into account any data processing or writes. For that reason, ATI has actually separated the read and write pathways on the X1800 and newer cards.