Regarding any comparisons of lenses which have not been tested by dpreview or DXOmark, go to someone like B & H Photo and read the user reviews. Both the Sigma 120-400 and the Nikkor 300mm f/4 have user reviews and these may be very helpful.

Prices may vary for 80-400 (1400 to 1800 as far as I know).
You may want to look at this:
http://www.bythom.com/80400VRlens.htm

you cant really compare 80-400 to 80-200 its like doable the range, and yes 400 one is a bit slow for sports photography etc but if you want just range with out speed its great ...
might want to consider 80-200 (thats what I got) with teleconverter (if you need a bit more range)

Thanks for all your feedback. Its hard to get best of both worlds - but I can see the logic of getting the 80-400 (exp. the 400 end on FX). I just need someones experienced testimony because it's almost as expensive as the 70-200. Can anyone hold their hand on their heart and tell me that it's a fantastic pro lens?

If you are after outdoor sport photography 70-300mm f/4.5-5.6G AF-S VR is actually very good lens, it is great up to 200 and very good to 300, it is very fast to focus and has second generation VR that is better and faster then one on 70-200 VR and will give you more usable range then 70-200.
70-200mm f/2.8G ED-IF AF-S VR is optically a bit better but is twice as much weight and has some issues in corners on FX that are exaggerated when used with TC and using TC will make it no better, if not worse, than 70-300 so it is not worth the trouble and it will only give you only 120-340.
You can even consider buying 70-300 and 80-400, 80-400mm f/4.5-5.6D ED VR AF is very good lens really and only a bit slower to focus, for the price of 70-200 alone and you will have much better set for sports and tele photography.

I would not consider the 70-300mm VR, I have seen it do some strange things when focusing. You should rent the 80-400mm then the 70-200mm with the TC of your choice before you purchase any lens.

I have had a great experience with these guys when I rented the Sigma 100-300mm.
http://www.lensrentals.com

Even on DX, 300mm is a minimum for bird photography if that's what you're using it for.

matstar You can also try the sigma 100-300/4 which is one of the best sigma lenses - fast and sharp and with a price tag similar to 80-400, as for 70-200 combo with TC17 on a FX body, I doubt it's a solution - 340 mm is fine on DX where You actually have a usable 510 but like I've said that's on DX, on a 35mm I found it a little to short (using with Nikon F100)

@ matstar : the el-cheapo is not worth it.. its even softer than the 18-200 which I have. I would consider the Sigma 70-300mm F4-5.6 DG MACRO instead.

Opps sorry .. The 70-300 I looked at was the one before the current G version. so I dont know if its gotten better or worst.

Hmmm you have a D700 right? then the 80-400 should be good for what you intend. cos the High ISO will compensate for the weaker aperture.(compared to the 70-200). The D700 will make the 80-400 a very different lens compared to a D80 with the 80-400.

Thanks guys! I mainly want to shoot outdoor sports - my kids swimming and motorsport. I also have a D90 with 18-200 which I find really good, but sometimes quite soft at the long end. The tc17 is a great idea for the 70-200, but then you lose 1.5 f-stop (which is still very good for 340mm).

Do you reckon I should buy the el-cheapo 70-300mm (AU$200) just until I can save up for the 70-200 + TC17 (AU$3500) ??

Hi matstar, The 80-400 used to be slow focusing.. because its glass is heavy and the motors inside the cameras were not as powerful as they are now. I have friends who have that lens and they report that the focusing speed had improved when they upgraded to the D300. They mainly use it for Bird and Nature photography. I assume that the D700 will have at least the same or more powerful motor. many people use the 70-200 with the tc17. That combination has many fans as well. due to the flexibility of the 70-200 F2.8VR and in combination with the TC it gets to 340 mm.

It really depends what you aim to use the lens for. (Assuming the price difference is not a concern)
The 70-200 is great for concert and events as well due to the F2.8 aperture. Whereas, the 80-400 range makes it a great street photographers lens and a decent birding and wild life lens.

80-400 speed is not so crappy, but also there's a speed difference between this and 70-200/2.8 but overall it's a great lens with a good price to IQ factor

Hi Everyone

I have a D700 with 24-70mm f/2.8 zoom, and need some advice for a second lens to extend from 70mm. I am fond of the 80-400mm because of the excellent range - or should I be get the 70-200mm f/2.8? What are the pro/cons with each lens?? Should I save my money and go with the much cheaper 70-300mm?. Another thing, is the 80-200 non-VR "beast".

The D700 performs excellent at high ISO, so maybe f/2.8 and VR is not an issue to me. Should I save my money and by the cheaper 70-300 f/3.5? I mainly want a walkabout lens, so the 80-400mm is tempting but have read that it's AF speed is really crap.