The 17-55 G lens is designed exclusively for DX format cameras like your D7000 and D90. If you plan to move up to FX Full Frame like the D3 and above and expect to use this lens, your going to lose sensor compatibility. However, this lens is going to give you the maximum compatibility with your cameras now because it's designed for the DX sensor in your cameras. It's also slightly better glass than the 17-35 D and in my professional opinion, handles metering and exposure better communicating with the cameras systems.

The 17-35 D lens is also excellent glass, has an aperture ring on the lens and can be used for DX and FX cameras. On DX cameras, your image will be cropped, meaning what you see in the viewfinder will be cropped so your final photo will slightly be trimmed down from the corners.

If your not going to move to a $2500+ camera, go with the 17-55 G as its made specifically for the cameras you have. If you are moving in the near future, go with the 17-35mm D, Although I would advise you to consider other FX full frame G lenses because they do such a better job interacting with the cameras software producing cleaner images that require less post-shooting processing.

Yes, but it has to be the AF-S series. They are the TC-20E II, the TC-17E II and the TC-14E II. They are expensive, but the only ones that will work.