Always 7 series is better than 6 series any day.
7600gs perfoms far better than a 6600gt is newer games in big margins whereas only in small margins or even same in older games.
In 3D MARK 06 u can see a big diff b/w them.
7600GS has exactly same no. of pixel shader units as in a 76GT, which is more than a 6800GT.