Also, keep in mind that the i3 (though still a dual core processor) has hyperthreading so it can juggle 4 processes at once... just not as adeptly as a true quad core... so that'll definitely give the i3 an advantage in some tasks.
If you're buying new today, i3 is the obvious path (or higher) ... if you've already got a C2D system, I'm not sure an i3 would be a worthwhile upgrade unless you plan on overclocking it.