Dual Core is just a term to refer to CPUs with two cores. The Core 2 Duo is a Dual Core CPU, as well as the Pentium E-series and Core Duo CPUs. By "iseries", do you mean i3, i5 and i7? It depends on the CPU. A higher-end Core 2 Duo will beat out a lower-end Core i3 or i5. Core i7 is currently the fastest CPU you can buy and is a quad-core, meaning it has 4 cores.