If I understand you correctly, your main complaint with the D200 is its poor low light performance. According to your profile, you only have the Nikon 18-200/3.5-5.6 VR which is a fairly slow lens. When I was using DX I got the faster Nikon 70-200/2.8 VR which aside from much improved IQ is two stops faster than the 18-200 at its long end. To get even better low light performance you could get a couple of f/1.4 or f/1.8 prime lenses.
With the right lenses, my old D200 still gives outstanding results. Your "new and improved" camera body will be "old and obsolete" within two years, whereas top level lenses will retain their value or even appreciate.