Difference between i3 and the core 2 duo's

i3 has much more cache (helps in games) and core 2 duos' are outdated, also i3 is quad core :). Intel are not going to make anymore processers for the 775 socket (what you put would put your core 2 duo in) so if it becomes to slow you cannot upgrade it without getting a new motherboard. Also check the website that 'GhislainG' said :)
 
Well basically it all goes down to the archtecture.

i3 processor is based on 32nm fabrication while core2duo is based on 45nm or 65nm fabrication
(32 is new and better)

i3 processors use the new LGA 1156 socket while core2duo's use old and outdated and dead socket LGA 775

i3 has 8mb cache will core2duo's have 6mb(correct me if i'm wrong in this numbers)

while both have 2 cores
 


Sorry dude i3 is not a quad-core it has only 2 cores both the i3-530 and i3-540
PROOF: http://www.intel.com/products/processor/corei3/specifications.htm
 

Woops, thanks for correcting me there :).