nVidia hasn't really explained it, but from my understanding the 4x.xx series of drivers gets much better performance out of the GF4s hardware, while the 3x.xx series seems to be a bit more stable on the GF3s.
At least for me in 9x/me the 3x.xx series is better, but the 40.72 set works pretty good too. On my XP side, I'm with the 41.03 set all the way.
I think the big dif 'tween the sets is the pixel shaders, the 4x.xx series seems to have much more quicker/aggressive programming in that department.
