What little I know of your needs, my first choice would be to get a 70-200 f2.8 VR which also gives the option of exploiting the various converters that Nikon provides if you need the additional reach and IF
you could get a 2nd hand one within you budget, that would be great. However, as it was totally out of my budget
, even second hand, I opted to go for a new Nikon 70-300VR which, although too short for my specific desires and cannot take the converters, provides a reasonable budget alternative with great results. On the DX models, it provides for up to 450mm and the VR provides adequate opportunity to slow the shutter speed down to compensate for the higher f-stop. As a general-use lens I am happy very with it.
You need to clearly articulate your own needs proir to any planned lens improvements/acquisitions and these will clearly differ from person to person. Are you in the market for lenses for wildlife and/or birding for only a few days/weeks a year (if so then rent what you need) or do you intend to make full use on an ongoing basis? I am also fortunate enough to have the newer technologies in the D7000 which allows for cranking up the ISO's when low-light photography is required. My next acquisition will be a bigger prime, if and when budget allows, something that may be better for you than the pro-level 70-200 f2.8. if your desire is for birding
I have had and used too many non-nikon alternatives without great results, so while many on this forum may praise the benefits of (for example) the Sigma 150 - 500 or 50-500 and I will not knock them in any fashion, my personal experiences will not allow me to travel that road again.... get the best you can for your needs (within your budget limits as you define them) or alternatively rent the best for the short time you may need them....
Keep your eyes open on the Nikon Press releases, for there have been rumours of them bringing out a 70-200 f4 somtime, certainly a lens I am looking forward to...