I think 17-55 is Great For D300.
No regrets.
But Get 2nd Hand is my Advice.
I think can get around 1.5K for good condition one.
You still have 1.5K to spare. Keep it and save it for rainy day.
Forget about the DX or FX Thing.
Unless you are very sure you will upgrade to FX in the near future.
If not then get the DX in my own view. No use foresee future too much.
By the time you wan to upgrade then decide.
Cause before you upgrade, you will know how much cash you have spare to burn, then from there you decide.
In my own view 24-70 although is good, but for DX, it simple not wide enuff. For 17-55, at least I find the range for usable in DX format. But like many say, go find out your most shot range and that will be your answer to your own question.