Anyone who has such hashing power has to choose between assaulting the network and participating honestly within it.  If said person has 51% of the hashing power, and participates honestly, he can capture 51% of the newly generated coins.  He could choose to reject every transaction, and simply publish empty blocks, but this isn't an attack on the network, really.  It does no harm, since transactions can still be expected to be processed by the miners who capture the other 49% of blocks.  However, such a person could choose to prevent all transactions, but to do this he has to turn his miners to the task of reversing all of the blocks that are created by other miners, so he would still only be able to capture about the same number of blocks over the same time period.  The effective hash rate of the network would be cut in half, but only if the attacker is successful at reversing all honest blocks before another honest block could be built upon it.  The odds of this continuing for any extended period of time is remote with only 51%.

No, an attacker with 50% of the hashing would not be able to take over the network.  Such an attacker would be able to prevent transactions from being processed, and potentially double-spend coins that he recently possessed; but would still be unable to do anything to anyone's coins that he had never actually owned.  The 50% mark just makes an attack possible, it doesn't make it easy.