I know the D700 is an FX format and my D300 is a DX format, but is the D700 a step up in overall performance over the D300? Better results using higher ISO's, truer colors, etc? Over the last couple of years I have been shooting mostly film in medium format and 4x5. I have it in my head that an FX camera will give me better low light performance and better prints...
I shoot with both. Can't say I see a difference in colors between the two, but high ISO performance is clearly better with the D700. I try not to exceed ISO 3200 with the D300, but ISO 6400 is my ceiling on the D700.
I have both, D300 and D700 and do not see any differences in a good light condition, but in low light it is a big deterrence, It is 1.5 stop at least, in high ISO D700 is much better and I do not see any noise at ISO 1600 when using D700. If you decide to go with FX you will enjoy D700.
I have both bodies and agree with the others. The color depth and dynamic range are identical. The D700 has about a 1.5 stop advantage in high ISO performance over the D300. Good Luck and Enjoy your Nikons!