If you need sharpness, and not megapixels or cleaner, higher ISOs, then you should notice a more immediate improvement with lens upgrades. Or perhaps a tripod for ultimate sharpness. And you'll need a new lens or two anyway if you do buy a FX camera. Generally folks talk about sharp lenses, not sharp cameras.
For what it is worth I have a D700 and no real interest in getting a D800/e. I simply don't need the megapixels.