Well, A 3.6ghz was the same price as a 3.2ghz dual core when I last looked. But since then you can get a 3.4ghz 65nm Dual core for the same price as a 3.6 single 90nm core, meaning in real world they will be identicle in single threaded apps. Sorry if I confused you with this.
Core duo is dual core designed with low electricital requirements to be used in laptops. It is better than a Pentium D dual core, and alot more efficient so can over clock well. Core 2 Duo is the better chip.