Generally speaking the current generation of Core i3/i5/i7 have about 20% more processing power than their Core 2 Duo counterparts.

Core i3:
– Dual Core CPU
– Hyper Threading

Core i5:
– Dual Core CPU
– Slightly higher clock speeds than core i3
– Hyper Threading
– Turbo Boost (it’s like an automatic overclock if the CPU is not too hot)

Core i7:
– Dual Core CPU (models ending with M)
– Quad Core CPU (models ending with QM)
– Higher clock-speed than the Core i5
– Hyper Threading
– Turbo Boost
– Virtualization (you can run multiple operating systems at the same time)
– Has circuitry that allows for easier and more secure remote access for corporate IT departments to trouble issues in a business environment.
– AE5 New Instruction set (No idea what this is)

Source:  http://www.tomsguide.com/