[pillainp]

I believe it is the "width" of the data path used to transmit data from the memory to the system: i.e. a 256-bit interface transmits 256 bits of data at a time, while a 128-bit interface transmits 128 bits of data at a time.

Therefore a card with 256 MB memory and a 256-bit interface is capable of transmiting and receiving data twice as fast (theoretically) as a card with 256 MB memory but a 128-bit interface.

Thus the first card would have a better performance index, all other things being equal.