Anthony, you and Neil can make room in that boat for me My D300 went to Nikon recently and I borrowed back the D200 from my son. I too go back to the D200 shots from 2007 and think they are better, in large part, but that was my first DSLR and I was treating it as if I were still shooting film so spent more time on getting the shots right without crop and PP!
I have the D3S and D700 (D300s/D300). Upgrading has not made me better at all, but life is easier mainly due to the speeds I can shoot in different light. I do not shoot Studio and no serious Landscape.
I think David's points are all very valid but I do not make much money from my gear so if I take NAS out of the equation it is a no brainer...no D4 and no D800 (at least for the foreseeable future )
I'll wait and see what a D400 offers and I am actively looking for a smaller travel/street kit. Some advanced skills training in Photography and PP would probably be a wiser move Cheers, Tom