From what I have read, an attack to the network does not need to be 51%. The likely hood to be succesfull with above 51% is higher. So it really depends on how you want to look at a secure network.
Although it is called a 51% attack it is really more a >50% attack. As long as the spawning genesis has more hashes and can be extended out to form a chain (or subchain) that can be accepted by the parent, then a 51% double-spend type of attack can occur.
Fundamentally speaking, any coin that has less than 50% of all of the possible hash for a given algorithm shoulbe be considered unsecure.
Well theoratically an attack can also be done with 1% of the hashrate. But as it is scaled logorithicallly, the chances of success at 1% are near to zero. So the higher the hashrate, the more likely the attack will be succesfull. As for being practical, once above 51% it is more likely to have a 6 deep confirmation.