Duke Leto
Without habitat, there is no wildlife. It's that s
I have been reading things that suggest that Nikon have improved the image quality on the D300s so, you could get a 300mm f2.8 VR and a 500mm f4 VR, before getting a D400 as the D200 is a very competent camera. If you get the 300 2.8, you could get a 500 f4.8 with a 1.7x converter.
Oscar
Gaz I do love these threads......
Very few people, me included have the technical ability to use a camera in the field as they compare them in a lab, I take shots with my D200 that are as good as my D300, the difference for me was 2 things the focus and the noise management, I have a better hit rate on the D300 than the D200 and i can up the ISO without too much fear of introducing too much noise (within reason).
I have not yet read anything that would suggest that the £500 hard earned would be well spent on a D300s unless you want video.
As for the comment regarding a 500/4.5 & 1.7x, this would be an extremely bad idea, it doesn't work and I only use it if I need to record a bird that is fairly distant. It will work okay with a 1.4x
In my opinion cos I have been through exactly the same process, stick to the D200, use the money that you would have spent on a D300/300s to get a good quality used Sigma 500/4.5 and spend a little on a 1.4TC and the battery pack for a D200 so its a better balanced unit.
With the lens you'll find an immediate improvement in your images without breaking the bank, and if you find it works you can always px the 200 for a newer body or px the lens for a Nikon unit.
Don't forget though the Sigma 500/4.5 is quite small for a 500 so make it a nice lens to lug about all day