This is an interesting question since there are so many possible answers. I can offer two, No and Yes. My first DSLR was a D70 (I still have it) which made me very happy for the most part. I did not like having a self-imposed limit of 640ISO. But my feeling is that if you can shoot within the D70's capabilities you will be rewarded with more than satisfactory IQ.
Did any of this make any difference to my decision to go forward, first with D300 and then D90? Nope. Those decisions were based on other characteristics of the cameras which had no direct effect on IQ but I feel do have more than tangential impact. RGB histograms is one such. The substantially more competent rear LCD is another. Better high ISO response is an important third. Live view is yet another. Every difference in capability between bodies can help or hinder the photographer in search of optimum IQ in any given situation.
So, did I experience a jump in IQ? For images shot at base ISO, no, not really. Well shot D70 images can be as punchy as you and PS can make them and if I were to limit myself to conditions that would make Ektachrome 200 happy I'd be set for life with the D70. Did I experience a jump in my ability to acquire potentially useful images across a substantially broader range of conditions? Absolutely. And that, in a very meaningful way, constitutes for me a jump in IQ. The D90 is my out and about workhorse, especially when out and about is inside.