The main and basic difference in the socket, the Dual Core and Core 2 Duo processors are socket 775 that are 65nm or 45nm depending of the model that u select, while the Core i3 and Core i5 are socket 1156 both Core i3 and Corei5 are 32nm.
775 socket use more power and produces more heat, also is more old than the new 1156 socket. The 775 socket are DD2, but this works with both DDR2 and DDR3, while the Core i3, Core i5 only works with DDR3.
Component price of the 775 socket is more cheap and the components like DDR2 are more cheap too.
Performance, all those have two cores, but not all those have the same threads number, the socket 775 (Dual core and Core 2 Duo) have two cores and two threads while the Core i3 and Core i5 have two cores BUT four threads.
Both Core i5 and Core i3 support Hyper-Threading Technology while the Dual Core and Core 2 Duo don’t support this feature. Hyper-threading works by duplicating certain sections of the processor—those that store the architectural state—but not duplicating the main execution resources. This allows a hyper-threading processor to appear as two "logical" processors to the host operating system, allowing the operating system to schedule two threads or processes simultaneously, here is where the thread power of the Core i3 and Core i3 is showed.
speed modes for the Core i5 600 series .
Core i5 650 - 3.20 GHz to 3.46 GHz
Core i5 660 - 3.33 GHz to 3.6 GHz
Core i5 661 - 3.33 GHz to 3.6 GHz, this chip has a TDP of 87W unlike the Pentium G6950, Core i3 & Core i5 600 range which have a TDP of 73W.