the future of DX

Can anyone here offer speculation about the future of DX and perhaps the benefits of DX over FX? With a DX, you can get a 350mm (equiv) f/2.8 for about $1600. With FX, the 300mm 2.8 will cost about $4500. Isn't that cost on the tele side in and of itself enough to keep the DX alive and well?

Of course, a camera like the D700 is king at something like ISO 6400, but other than that and the use of a lens like the 14-24 and fast wide-angle primes, will the DX crop factor keep it viable, even for a professional (or semi-pro) sports or nature photographer?

Also, how much better actually is a FX sensor if you are looking at say 24x36 inch prints at a lens like a 50mm lens? Does anyone have any side by side comparisons of actual shots from DX and FX of the same subjects?

One last question...I find that my D50 gives me great results at ISO 400, servicable results at 800, and not so great results (but very usable at 8x10 and under) at 1600...what can I expect out of a D300?