The texture files would be a lot bigger in next gen along with the other advanced gfx there probably is a lot of disk swapping going on, i would have thought having the game on the same drive as the O/S is a better but personally i have all my games on a seperate drive (O/S drive is quite small!).
Dont have the game myself, still tempted to get it though but i wont be happy not being able to run next gen well.