i3 is Intel's latest version of dual core desktop processors. They employ 32 nm size, an integrated graphics core, & hyper threading. Includes i3 530 & i3-540.
The Core 2 Duo was Intel's older twin core processors, usually with 65 nm size, no on-chip graphics or HT.
I have an i3 530 and it is faster than any core2 duo I have used, overclocks like crazy, runs cooler, and is much cheaper. Some of the core2 quads are faster, but they are double the price and then you might as well buy an i5 750 for $199 which beats the crap out of any core2 or i3. Is this for an upgrade or new build? The core2 series uses the old LGA775 socket, while the i3 530&540, and the i5 750 use the newer LGA1156 socket.