The Pentium D was a decent performer at the time, and a major step forward as dual core processors hit the mainstream. One of its limitations was that Intel literally just stuck two chips together on one die, and the two cores did not have direct communication (AMD's X2 chips, including the popular X2 3800+ were the first dual cores that communicated directly).
At this point, the entire Pentium D is totally out of date for anything, with performance that a modern Atom can beat.
For a gaming box, the Pentium D will definitely need to be upgraded to whatever the budget allows, and the performance will be much better.