Current graphic cards in Mac Pros can run (3) 30″ or smaller monitors. They have (1) DVI and (1) Mini Display Port, they are active at the same time.
For the second monitor you need to buy and adapter to go from the cards Mini Display port to DVI, the 30″ adapter is $100, 24″ are $30 (i think).
Two 30″ monitors would be huge, I would need to send out a search party to find my curser.
You cannot use (2) of the new Mac 24″ LED because there is no adapter (yet) to go from the monitors Mini Display connector to DVI on the Card.