Think best route (although new body is v tempting), is to get more glass, it would be a better investment I believe, so will start looking at some primes I think.
Current lenses are :- nikon DX 35mm 1.8, nikon DX 18-200 vr1 3.5-5.6, nikon 28-70 3.5-4.5 D, sigma DC 17-70 2.8-4.5 macro.