If you want much better quality, then I'd suggest looking at the Sigma or Tamron 70-200mm F2.8 (the Nikon version is better, but a lot more expensive). This would be basically the same focal range as your current lens, but with a much larger aperture (a good thing) and better overall quality.
This lens is just over $500. You can get a non VR 70-300mm for much less, but it's not a great lens.
If you want a longer lens, have a look at the Nikon 70-300 VR. From what I've read, it's a pretty good lens...not outstanding but not terrible either.
There is the 300mm F4. Probably an outstanding lens but it's $1400.
(The 300mm F2.8 VR is $5300)
You won't find a *good (or even OK) 500mm lens for a reasonable price.