I thought the official word was that, for now, TXAA will only be supported through hardware (680) and it would be on a per game/engine basis. Then it will be supported by the 500 series later on through software.
You can only get real hardware MSAA in a deferred engine with DX10/DX11. And even then you take a pretty sizable performance hit without cleaning up all the jaggies. So Cry 3 will likely include a healthy mix of some modern post processing AA techniques.