Oldcolt,
If you raise the front sight only, that means the gun will have to be held so that rear is raised up for the sights to align. If you hold the rear of the gun higher in relation to the front, the gun is pointing lower, which would cause the bullet to hit lower yet.
Edit: The reason that these guys sell a taller front sight to go with the rears is that the adjustable sights tend to sit higher than the standard fixed. If you were to install a higher rear adjustable with the standard front, the gun will have to be held so that the front sight is higher than normal to line up with the higher rear. This normally means higher point of impact.
[This message has been edited by James P (edited 10-08-2001).]