When Ral Zarek's second ability resolves, the next target player in turn order (or, if it's a target player's turn, that player) chooses a card in hand without revealing it, then each other target player in turn order (if any) does the same. All chosen cards are then discarded at the same time.

If a card in a graveyard has {X} in its mana cost, X is 0 for the purpose of determining its mana value.