Sure it is, having a 256bit bus doubles the memory bandwidth (compared to a 128bit bus). It's like going from SDRAM to DDR, it doubles the theoretical bandwidth. Of course that doesn't equate to double the FPS but it does mean more FPS. Memory bandwidth is especially important when you use AA, it will lessen the performance penalty. How much improvement you will see depends on the game, a game that is heavy on drawing ops (ie Doom 3) will benefit more from a faster core then more memory bandwidth but it will still help a bit.