using a Pentium D wouldn't be worth the money. you would end up spending MORE in the long run on that than you would with a C2D 6300 OC'd. The D would run hotter, consume more power, not to mention the performance difference.
EDIT: It wouldn't be worth it if you had to BUY the Pentium D, but it would be an option if you already had one... but it would still run hot, and consume power. /EDIT