Can I suggest a good 400-500mm lens? Yes.
Do they zoom? Not really. Most are fixed-length prime lenses.
Can you afford them? No. (Can I afford them?! No! Hard to justify the costs for me!)
For example, the Sigma 50-500mm for $1,600:
http://www.amazon.com/gp/product/B00...SIN=B003A6H2Y8
The 300mm lens is the best length you'll get as a consumer, at least for consumer prices. Although I would note that a 300mm length lens, measured as it is by 35mm "full frame" measurements, is more along the lines of 450mm when mounted on crop-body (APS-c) format cameras, like the D7000 and my own D200.
You can't hand-hold 300+ mm and get a quality photo easily. Those lengths must be monopod mounted at minimum. But even then, a top-notch pro that knows how to be still will have issues keeping it steady. Your really have to get a tripod for 300+ lengths.
Even 200mm should often be put on a monopod. I'll admit to cheating at this quite often, preferring the freedom of movement without a "third leg" to get in my way. But I often find myself regretting those monopod-free decisions (when shutter speeds dip down below ~1/350th).
It's easy to make blurry photos when you're zoomed out to that tiny fraction of view. And contrary to what I consider mass ignorance, "VR", "OS" and "IS" won't save you. Even those various anti-shake techs are only good under certain uses -- and this isn't one of them! It's hard to fix handheld shake at 250mm+ range photography with anything other than old-fashioned monopod/tripod stabilization (or those fancy "steadycam" type accessories).
There are some 500mm mirror lenses out there, but they're not for the casual photographer. I don't much care for them myself, to be honest. You are basically shooting at f/8 and the image tends to be a bit soft and hazy. I shot with a Vivitar 500mm mirror lens back in high school for fun, but that was a long time ago. I'd probably hunt down the same 1970s/80s lens and body again, and shoot film, if I wanted to do mirror work these days.
For example, the Opteka 500-1000mm for Nikon:
http://www.amazon.com/gp/product/B00...SIN=B001GKNNDY
At $140, it is quite cheap.
For my own 300-600 needs, I opt for a TC-14E (1.4x adapter) or TC-20E (2x adapter).
However, these may also run at a much higher price than your current budget (as per other threads) will allow.
And they only work with certain lenses (AF-S and AF-I Nikkor models).
Hope that helps.