the quality of images regardless of long exposure or portrait shots

I have been into photography for some time now, and i am trying to upgrade to a more pro level camera without spending 3k+ on a pro series DSLR. I shoot a lot of long exposures and night shots of architecture, and cityscape's, but i also shoot people (portraits, models, action, and street/club photography.) I am looking for a camera that will perform in any light level, and still deliver a quality image that my clients will expect. I have a friend who shoots with a D7000 and loves it. I have used a Canon with a DX sensor for over a year now, and i would like to get a Nikon with an FX sensor, so i can make larger prints without getting a really grainy image. my only concern is the lower pixel level of the D700 (12.1) vs the D7000 with 16.2. my dream would be a D4 or somthing like that, but lets be real i have neither the funds or the need for a camera of that magnitude. If someone could give me some insight into what i should do in this predicament i would greatly appreciate it!