I concur with what Brian said. I too had the same thoughts... meaning I had some bit of cash and was thinking of either getting myself a D90 or get more gear. When I had that thought I had 18-55mm (the non-VR kit lens) and the 18-70mm (also a kit lens). Then I bought 70-300mm (VR version), SB-600, etc. My last equipment I bought was the Kenko extension tube set (a cheap man's macro) to see if my D40 could take nice macro shots (it did).
My actual first macro shots (took me 30 mins to come up with an idea and to set it up) was a couple of transparent dice I had since I was a kid (there's a picture of it in my gallery) but yeah, my point is to concur with what Brian's advise is.
Yes, my noise is higher when I shoot at higher ISO but with Nikon Capture NX 2, shooting at RAW, it reduces a fair bit (yes, it's not as clean reduction as Noise Ninja or some other noise reduction software but I'll get one of those soon... after I get my next item - Sigma 10-20mm).