I also have the first version of the 70-200 and I absolutely love it to pieces, as the new one is out you may be able to find a bargain too. I'm sure it could take a 1.4* TC with ease and on brighter days a 1.7* too, I wouldn't bother with a 2* though. I use my 70-200 throughout the zoom range and find 200mm to usually be enough on DX, on FX I'm not so sure. Perhaps if you want the reach you could look into 300mm primes (Nikon 300mm f/4 or equivalent). If you wanted to choose between the 70-300, the 100-300 and the 120-400 I'd go with the 70-300 for sure. I've heard the 120-400 is so bad some lens rental places don't even rent them out any more!