These kinds of stats always must be taken with a grain of Salt. We wont know til we see it
Real world results usually end up being somewhere in the 25-30% at most performance increases over previous models, but more than double would be amazing!
I know Nvidia has been struggling, and not doing well with profits since the ATI/AMD 4000 Series was launched. But they still have the most powerful GPU today.
I can only hope that the GT300 is like what the 8800GT was... a HUGE upgrade to just outright serious performance, and the 8800GT was the king for a good while there. I can see the new GT300 doing something similar and just bringing graphics to a new level once again.
We Shall See. I hope it doesn't cost to much, but as long as ATI keeps making good products as well. The competition will stay strong, and the prices will fall. In the end that is good for us, the consumer. =)