The Mark III 5d achieves low noise with more aggressive noise reduction in the camera. The images are a little softer - especially as ISO moves to high levels.
The nature of a DX sensor is the pixels are smaller - and thus noise is going to be higher than FX. You also don't have the downsizing benefit of FX. But the technology of lowering noise continues to improve in every generation of cameras. I think its safe to say that a D400 will be the best DX camera for low light if it materializes as expected.
There are a lot of new technologies coming. The big benefits of high ISO noise reduction have already been achieved. There is a lot that can be done with focus and dynamic range with similar major steps forward.