hans, this is obviously not correct. 
In computer world only the lowest hardware logic/gate level deals with 
bits. To OS and software/application/games, they organize bits into 
bytes for easier management. The minimum unit they will transfer is in 
byte. The situation you described will 
never happen in the real world unless upon hardware failure. Simple as this:
128 bit bus width: can handle 16 bytes/words at the same time
64 bit: only 8 bytes/words at the same time.
Giving the highly sequential nature of video graphic processing, having wider bus width gives extremely favorable bandwidth increase.