(1) According to the meaning of the question: 400×80%=320 (yuan), 600×80%=480 (yuan),

Xiaoming bought 400 yuan's books with this discount card, and he spent 320 yuan. Xiao Fang bought 600 yuan's book with this discount card, and she spent 480 yuan.

(2) It will be more economical if he buys at least X yuan of books.

According to the meaning of the question: 100+80%x≤x,

Solution: x≥500,

Then at least it will be cost-effective for him to buy 500 yuan's books;

(3) According to (2), when x is less than x 500 yuan, it is more economical to buy a discount card.

So the answer is: (1) 320; 480