Core 2 Duo is old and runs on socket 775
Core i3 is new and runs on socket 1366
Because the Core i series are based on newer, improved architecture they're generally a lot better than the older Core 2 chips.
I can't really say more without going too technical (not that I's an architecture wiz anyway).
Suffice it to say that Core i3 is probably better than all but top-end Core 2.
EDIT: my bad - Core i3 is socket 1156