Why not use the money you'd spend on the D300, on a 1.4 converter?
The higher ISO capabilities of the D3 will compensate for the loss of light.
I mean, if you KNOW you're gonna get the D3 anyway, might as well buy great lenses, instead of another body, and use them on an amazing body that allows you to shoot in relative darkness.
I don't know the price of the D300, but i'm pretty sure you can squeeze in a teleconverter and a great tele prime in its place.
Unless you already have all the lenses you could ever want...
Well, I am shooting with a 70-200 f/2.8 VR for football & volleyball, and I already own the TC-17 teleconverter, which loses me two stops. I suppose I could go out an buy a better lens... as soon as somebody invents one... While my Nikon teleconverter is sharp, it is not as sharp as the lens without it.
I am shooting the Sigma 24-60 f/2.8 for basketball, and while this works OK I am going to replace it as money permits with a good Nikkor.
I am not really thinking too hard about dropping 7 grand (or however much it is) to buy the new 400mm f/2.8 VR so I can shoot high school football for the local newspaper and the regional sports websites that I shoot for. Sure, it would be nice to have, but that is indeed a chunk of change.
In most of our local gyms & football fields, I have to shoot at 3200 ISO ALREADY to get 1/800th of a second at f/2.8, since the lighting is so poor. Losing 2 more stops is NOT the answer, nor is shooting slower and having blurry pictures.
Shooting High ISO and using Noise Ninja (which I absolutely LOVE by the way) is what I do already... and 3200 on a D80 is not a pretty sight.
Because of the sensor resolutions, it takes 1/800th to really stop action on a DSLR when it comes to sports. The D3's full frame sensor would work just fine indoors for basketball (obviously), but not at all for football under the conditions I am shooting with the lens I am shooting. (night, badly lighted fields, no sideline access between the 30 yard lines -- press not allowed into the players box).
Friday Night Lights may be a cool idea for a TV show (dunno, never watched it) but let me tell you the actual lights these schools use are pitiful.
I will end up getting a D3 eventually, but I don't NEED one right now. When I NEED one, and happen to have a spare 5 grand sitting around, I will buy one.