Duplicate Digit Determination III (Posted on 2010-12-23)

(I) Each of x and y is a positive integer with x < y such that, reading from left to right, the last three digits in the base ten expansions of 1978^{x} and 1978^{y} are congruent.

Determine the minimum value of x+y.

(II) What is the minimum value of x+y - if, keeping all the other conditions in (I) unaltered, the last four digits in the base ten expansions of 1978^{x} and 1978^{y} are congruent?