D90 and D5000 share the same sensor and image processor. How did that translate to the D5000 having better iso performance? If you have read from somewhere that explains or shows this in a controlled test environment, please share.
As for the comparison of D300 vs D5000, posted by KRW, D300's iso performance is actually much better than the D5000 up till 3200. At 6400, it loses some detail due to the NR algorithm. That said, if you take the RAW taken at 6400 iso (or turn off NR) from D300 and D5000, and process NR yourself, they are about at par.
Personally, I seldom go over 1600 iso when I shoot. Only once did I push to 3200. And I never had to do 6400. There you go.
BTW, don't believe everything KRW says. Some of the stuff he says I find are rubbish. But still, he is an entertaining read for laughs. If you want to read good solid reviews, read this one
http://www.naturfotograf.com/index2.html