Psychology student struggling with probabilities here.

Can anyone please tell me how do I calculate:
1) The probability of getting at least a 501 when rolling 2 1000-sided dice (or when picking balls out of a sack with 1000 unique balls one after the other with replacement)? i.e. getting (501, 642) or (12, 732).
2) The probability of getting exactly 500 when doing the same thing as above? i.e. getting (500, 790) or (4, 500).