The these both processors have only 2 cores, the major difference is the bandwidth.. i.e the speed the core 2 duo offers is enourmous than that of the dual cores... The C2D has more room for Level Cache. Apart from Dual Cores and C2D, Pentium D is nothing but, two prescotts running together side by side.. Its very HOT! So u cant expect more over clocking... but the C2D is made with revolutionary 65 nm fabrication process.... which runs cool and good for overclocking...
Please correct me if i'm wrong