The D7000 has a 1.1 stop noise advantage over the D50. The CMOS sensor is also a nice step up in IQ. Dynamic range is 3.1 stops better than the D50.
If anything, the D7000 will really test the resolution of the 80-400mm lens.
One word of caution with the D7000. If you're going to use it with that lens, it would be best to use a tripod. The D7000 is more motion sensitive than the D50 because of the density of the sensor. With a D50 you can hand-hold the camera with a shutter speed of 1/focal length of the lens. With the D7ooo you have to use a faster shutter speed or a tripod so the formula for hand-held shots is 1/(1.5 x focal length of the lens).
That means that a hand-held photo with the 400mm lens would need to be at a minimum of 1/400th with the D50 and 1/600th minimum with the D7000.
You'll really enjoy the upgrade.
"Red is gray and yellow white, but we decide which is right ....and which is an illusion"