Mon 14-May-12 03:46 AM | edited Mon 14-May-12 03:48 AM by ShrimpBoy
I agree with Hendrik. I also went from D70 to D90. High ISO improvement was obvious, but otherwise (occasional weird cyan skies aside) the D70 was just as good and more forgiving of poor technique when you pixel-peep for sharpness.
The D90 is still a nice step up from the D70 though. It shoots faster, has a bigger viewfinder, has a bigger rear screen, has a more advanced focus system (if you care about that), doesn't lose color when shooting high ISO, doesn't produce as much noise at equivalent ISO and goes two stops higher in ISO. And RGB histograms as Hendrik pointed out, live view for more accurate focusing, video if you want it, etc, etc.
Having said all that, you'll probably get more bang for your buck putting the upgrade money into something like a Tamron 17-50 f/2.8, depending upon what you like to shoot.
Edit to add: I'm trying to sell a D90 if you want one. But beware, it's a slippery road: I've already fallen to the temptation of a D7000.