The D700 and D800 are both great cameras, but so is the D300. There are certainly some specific attributes that one may want with those cameras, but there's really no guarantee for much better pictures. Too much depends on you, the user.
Is there something about your results now that you're not happy with now? If so, try to isolate what it is and what feature of a new camera will help you overcome it, and make sure it's not just something you can improve with your own efforts. There ARE reasons to upgrade, but you'll make a much better decision if you know why and when you should. I've seen countless outstanding landscape shots from DX format cameras. Generally speaking, that should not be a factor.
Now, if you want to crop a lot, that IS a factor and supports the extra mps of the D800. What about shooting in low light and using high ISOs? It's the reason why I upgraded from the D200 to the D700.
Of course, if you just want a new camera, well, that's okay too!