Intel Core 2 Duo is Intel's new line of processors. Simply put, they are amazing. For WAY less you can get a processor that outperforms the AMD FX-62 (AMD's best proc.) in everything, including gaming. It is defintily the better option.
Pentium D is Intel's older line of dual core processors, they are good for multitasking but not for gaming.
Core 2 Duo is good at both and is by far a better processor.