Everyone should be buying core2duo's. They do outperform the dual core processor quite a bit as long as you don't buy the lowest version. To give you an example, a quad core 2.4 is about 2x as fast as a dual core 3.0. Honestly, the sweet spot in price performance is a quad core now.
http://www.tomshardware.com in the processor section has a tool that lets you view performance differences between all of the consumer amd and intel processors that is a pretty good guage to use
Cheers,
Brad