Previously NVIDIA said that its Fermi (400 and 500 series), Kepler (600 and 700 series) and the new Maxwell (750 and 900 series) GPUs would all support DirectX 12. However, an updated support document confirms that the older Fermi series are still using the WDDM 1.3 drivers, rather than the new WDDM 2.0 drivers needed to support DirectX 12.
The first game that will support DirectX 12 is the sci-fi RTS game Ashes of the Singularity from developer Oxide Games and publisher Stardock. The game is already available in a very early alpha version for people who pre-order it. It will come with a DirectX 12 benchmark as well. More DirectX 12 games, including Fable Legends, will be released later in 2015.