First the easy question - is the FX chip significantly better than DX. The answer is no, but it depends a little on your planned use. The DX cameras are sold in much greater volumes as tehy provide light weight, lower cost, and pair with lower cost DX only lenses. The DX crop factor has some beenfits for situations where longer reach is desired. There is nothing wrong with a DX camera - and you can look at images in the National Geographic taken by Bob Krist with his D90 for proof (the Image Doctors podcast early this year with Krist was excellent).
The D7000 is roughly the upgrade to the D90 - with many features that equal or exceed the D300s. It is an excellent camera with the latest features and technology. With the DX body you can use DX lenses providing a lot of flexibility at lower cost and lighter weight than the FX alternatives. I own a pair of D300s and have ordered a D7000 to replace the D300 used by my wife. It's probably a good upgrade from a D5000.
The D700 does not seem to fit. It implies a completely different path into pro level lenses. While you get a full frame, you have roughly the same ISO performance as the D7000. For lenses you will need to replace all DX lenses with much more expensive FX models - each of which costs $1000 or more than their DX counterparts. What's more, the D700 is priced near $2000 because it is near the end of the product lifecycle. There will likely be a new model replacing it in some form or fashion before long. I just don't see any advantages of the D700 right now unless you already have the lenses for FX - and even then I can't think of a compelling reason (there are a number of small advantages with frame rate, AF system etc., but nothing truly compelling). I am planning on an FX camera (waiting for the successor to the D700 or D3), but I already have the lenses and will keep my D300.
You suggested a 24-120 lens with the D7000. I think the 16-85 is a much better choice to go with both the camera and the 70-300. I have a 24-70 and can tell you that 24mm is rarely wide enough on a DX body. I use a 12-24 and a 16-35 regularly, and would not carry the 24-70 without an ultra-wide. I don't hav ethe 24-120, but the reviews I have seen indicate the image quality is not as good as some of the alternatives. The 16-85 /70-300 would make a terrific kit - and is one I own.