Basically, when you send data, typically, nowaday processors are 32 bit processors, meaning that there are 32 "threads" of binary flowing through it at once. Before (I think its 2000 anway) windows 2K, the Windows OS' were programmed to utilize 16bits of whatever amount your processor could handle, thus, thats why win95, 98, 98se, ME, and ther others before it, were SO SLOW. Now, addressing your hard drive. I believe that most fairly old hard drives run at 16bit....not sure so don't quote me on it. If the harddrive you have, supports 32 bit (if it doesn't and you enable it, it SHOULD just default back to normal) then essentially, you get more data from your harddrive.
Hope that helps!