My guess is the D400 will have the same size photosites and pixel density as the D7000 and D800, but a DX size sensor. What it will likely add is write speeds of the D800/D4 which means 10 fps for 16 megapixel files. It will also offer improved AF of the new generation cameras. It will also offer video of the D7000 format and quality. Now this is all speculation - just applying exisitng standards to a D300 successor.
For wildlife there are tradeoffs. Increasingly I've found that using a longer lens to blur backgrounds has advantages, so the FX sensor is better if you can fill the frame and if you have enough shutter speed.
If you lack reach, the crop factor of DX becomes more important. The key is the amount of pixels you put on the subject with your ending image. In that case the D800 with a DX crop is equal to the D7000, so you are not really loosing anything.
Now the wild card in all this is the amount of light. In theory, you need 1.5 times the focal length for DX shutter speed, but 1x for FX - unless you plan to crop to DX format where you need 1.5x just like DX. But more important is that for birds, I want 1/1000 sec to freeze motion regardless of the camera. That's where the D4/D3s/D3/D700 still have an advantage in that you can increase ISO by 2-3 stops for equivalent noise. That means with low light conditions of 1/250 sec, I could increase ISO and still get my 1/1000 sec shutter speed with a D4/etc.
The other wildcard is the impact on noise of downsizing the image. With DXO's testing, the impact of sensor size is neutralized by standardizing the image size at a smaller size than the camera generates. You can see the impact with a D3x/D300 comparison. That means noise gets smaller and is less of a problem. So if your goal is 12x18 prints, you could easily downsize a 36 mp image and eliminate a lot of the noise of higher ISO levels.
Finally for wildlife, some photographers shoot large bursts at critical activity. You won't get that with the D800. Even in DX crop mode, it is relatively slow. If your style is to shoot bursts, the D4 is the champion.