[cyclohexane]

Don't use the 2x with the 70-200. The AF gets excruciatingly slow in anything but the best light. I was using a D2H, and even it hunted like crazy. For small web use if you oversharpen a little the images should be usable, but I'd be vary of trying to make prints. At 400mm, the image quality really suffers; you're not mounting a TC to use the shorter end of the zoom, which actually perform okay.

You could probably get passable results stopped down to f/11-13, I can't remember the last time I had enough to light to shoot something that I also needed a long lens for at f/13 at a low ISO while keeping my shutter speed up to freeze action.

I've heard of success stories with the 1.7x, but really I wouldn't use anything other than the 1.4x with the 70-200. 280mm f/4 isn't a terrible long end, and you get a 1 stop light advantage over your 70-300 at 300mm.