Unless the camera is keeping you from some certain kind of work you want to do I myself would invest in other photographer's books, ie: Ansel Adams, Ralph Gibson, Garry Winnogrand or what ever photographers' work you like. This will improve your photography a lot faster than a new camera. A great learning book is Looking at Photographs by John Szarkowski. It would be an extremely rare thing to find no other photgrapher's books in really good photographer's home.
Better cameras do not make better pictures, growing photograpers do. I mean this to be helpful, it really deepends on your goals as a photographer or a person, you may have no interest in looking at picture books or already own a bunch.
On cameras if I had many lenses of 1 company I would not jump ship, too much hassel. Nikon, Canon what ever more or less the same, especially in a digital world. When I buy a digital camera I look at it as a refridgerater full of film and remember ultimately it is disposable so what are the dollars and cents as it relates to my shooting habits. These days the cameras get improved slightly often but I find it helpful to wait at least 2 generations before I bother to pay much attention. I can vouch for the D90 it's the plastic version of the D300 both of which are the FX versions of the D700 and D3 which are both DX meaning full frame.
Out of my posted images here: 1st image 1977 was from a used Olympus 35mm film camera, 2nd 1987 was from a Nikon point and shoot 35mm film camera, 3rd 2005 was from Nikon D70 digital w a $120 70-300mm lens, 4th 2008 was from a Nikon D3.
I have a 20x30" print here from the 6 megpix D70 that I did all the darkroom work on in PS that looks incredible. There are lots of things to consider, I just wanted to point out there are more options than you may have thought about.
The best time I ever had was going to a photo workshop in Vermont in 1980, if you have never attended one Maine Photographic workshops are fantastic.
Hope something here is helpful, if digital had not shone up and I did not shoot weddings for a living I would still be using 2 Nikon F100 film cameras until they broke and could not be repaired. If I never shot a wedding or any paying photo job I would still be using my used Nikon F2 with my hand held light meter. I would not have bothered with any of it unless I needed it.