Take a look through kenrockwell.com, who has helpful information for making these kinds of purchase decision, as well as advice for making better pictures. You are not likely to go wrong with either Nikon or Canon (and in fact, the camera you have is likely to be able to take excellent photos). The differences in quality are negligible. Most important are your shooting practices, and it sounds like you have some experience built up already.

Since you have some lenses, you might want to think Nikon first, so you don't have to start from scratch. If you are dead set on upgrading, and want something closer to what the pros use, a Nikon D7000 is as close as you can get, until you move to full frame, which will also cost you considerably more. The D7000 has all the bells and whistles you could ever need, an excellent sensor, very good autofocus and metering. It is also currently difficult to get because of the production issues in Japan and elsewhere. Read read read for awhile, make sure your needs are very clear in your mind, and the right camera will emerge from all of that.

Good luck!