Honestly, if the 5DIII isn't rocking your world, the D800 isn't all that much of a step ahead. That is, if the 5DIII is inadequate, the D800 won't be the solution. It's a step in the right direction, sure, but not a very big step.
For something that solves the resolution and dynamic range "problems" of the 5DIII, you'll need a bigger format. And you're not looking at the bottom-of-the-barrel entry-level obsolete stuff in the bigger format, either.
If I reach the point where 135 format won't do it for me any more, and assuming that I've got the money to do so -- and, realistically, if you're making money off of this, the only kinds of clients that demand more than what you can get from 135 aren't going to blink an eye at the fees you should be charging to go to medium format -- then I'd leap straight to something with a full-frame 645 sensor. Body alone will be as much as a Great White, maybe more.
But, again. Either you're charging your clients at least a few grand for the day's shoot and so you can afford it, or you really don't need that kind of gear (however much you might want it).
One last thing: unless your printer takes ink by the gallon and paper by the yard, don't bother. If your printer sits on your desktop, or if you don't print at all, you'll never see the difference. And I'd include all but the very top end of the mass printing market in that, too...a 5DIII is already overkill, as far as image quality goes, for any magazine you'll find on the newsstand, including National Geographic and the fashion magazines. Hell, it's even good enough for Arizona Highways, though it's been a while since I've checked to see if they've even started accepting digital at all....