DX to FX
Here's my take on it:
I went from a D300 to a D800. I wasn't sure what to expect, but the low-light quality of the newer camera is huge over the d300. In fact, I would say tha almost any newer prosumer or higher camera will have much better low-light capabilities than the d300.
Low ISO & daylight pictures, I'm not sure if there is a huge visible difference at regular print sizes.
What I think you will notice though, is that you effectively get much wider with an FX camera than with the d300. The lenses are more expensive, but I can see how for architectural shots, this could be quite useful.