If you're already running C2Ds, don't worry about upgrading to an i3. That's like spending the money to buy a car that gets 37mpg when you already have one that does 35mpg.
As far as the pentium dual cores, they are trimmed down C2Ds. You'll see they are listed as Wolfdales (45nm) same as the 7xxx and 8xxx C2Ds. They just are limited to slower FSB speeds and less L2 cache. They are great overclockers still and for the money are great CPUs.
Heres the E7200 vs the E5200. The E7200 is the better chip, but spending almost twice as much might not be worth it. Considering both will overclock near 4ghz, you get more than just 2xP4s worth of performance out of the PDC.http://www.anandtech.com/bench/default.aspx?p=62&p2=66&c=1