After Intel's Netburst (Pentium 4) architecture became a serious liability to their competitiveness, they looked towards their Pentium M mobile processors for inspiration. The key here is efficiency, rather than brute clock-speed. Intel then refined their designs and the Core Solo/Duo series was released. Effectively Intel fell into a hole they dug with their 'higher clocks = better' marketing deployed during the Pentium IV days. I'm sure the AMD execs were giving Intel the 'fucking idiots' grin there. Core2 Duo is a further refinement of the Core Duo range. Current range is:
E6300 - 1.83GHz
E6400 - 2.13GHz
E6600 - 2.4GHz
E6700 - 2.66GHz
X6800 - 2.93GHz
There are three additional Core2 Duo processors on the way as part of Intel's first iteration Conroe refresh.
Intel also plans to release their Intel Quad (formerly Kentsfield) series soon too, boasting essentially two Conroes slapped together (non-native quad-core).
Current models planned include:
Q6600 - 2.4GHz quad-core
QX6700 - 2.66GHz quad-core