There is only going to be one standard for the 360 games, and it will be highly unlikely that there would be a performance boost with hdd's over games that ran without it.
Bottom line is, 360 game will not use cashing or virtual memory if the hdd is not required for games to work. Simple as that. Which, imo, is a waste of an hdd, and the main reason to have an hdd. Being able to use the hdd for game performance would be a big plus in the wars against the ps3.