I've been seriously shooting birds now for about 3 years. I use a d300s body and select from three lenses: My primary is the 200-400 f4 vr, second choice is my trusty 300mm f4 prime and I have a 70-200 F2.8 if I want to capture more environmental views. I've found that the frames per second works for birds in flight (especially in keeping wings from blocking the eye) and the images are clean, sharp and detailed even at 800 to 1250 ISO. In comparing my images to others I've seen I am satisfied with my results. Like slopoki noted, I'll shoot the 300s until the shutter dies and then worry about an upgrade.
I was treated to the use of a d800 one weekend and did not see enough improvement to warrant a purchase. My 300s and I are best buds, so far the force has been with us.