Clock-wise.. the i3 is a dual core. They're about the same speed. However, the i3 supports Hyper-Threading, which means each core can do 2 things at once, instead of the normal Core 2 Duo which can only handle 1 thread per core.
What applications do you use your computer for? There really isn't a big boost from a Core 2 Duo to the i3 for the majority of users, imo.