Pretty much Fermi is just too little too late. I mean- release it at the same time and maybe people would have been happy, but release it 6 months later and only win by a small percentage in performance and lose BIG in power and heat and some in price as well.
Problem is that you will never get a driver that plays great with all games, they optimise the drivers for the main games at the time.
In general i think there is plenty to come and if they latch on to a specific game and optimise for that then i dont think double figure % improvements are out of the question.
ATI has won 2010, they'd won it as soon as midnight Jan 1st hit the world. Nvidia can only hope to get a refresh of Fermi on a smaller fab process by 2011 to begin to gain ground on ATI factoring in ATI's pretty much guaranteed refresh.
ATI's HD5890 (if there is one) will directly compete with the GTX480, most likely beating it. Considering the HD5870 is only ~10% from the GTX480, a 20% increase in performance from the HD5890 is reasonable.