i3 has hyperthreading, and is made of a smaller die than the 65-45nm core2 duo. It is a 32 NM chip and will supposedly OC bettwe at lower voltages. It is basically the same interms of preformance with a slight increase but not enough to warrent an upgrde from a core2duo to an i3.
I'd say it's equivalent to E8400, maybe ( a big maybe ) a bit faster.
It IS faster in multi-thread apps though because hyper-threading
And it's little cooler and OC's little better cause the 32nm process
But if you're looking for upgrade from e8400 to an i3, it's not worth it. Better look into something like i5 750 or PII X4 955;)