I'm sure you've already bought it, but here's my opinion:
Pentium Dual Core is Intel's first attempt at dual core. I believe it was actually two dye instead of the common one dye nowadays. Both generations use the same socket. Socket 775, if I'm not mistaken. What you would gain from having Core 2 Duo over the Pentium D is pretty expansive. Faster speeds, higher front side bus, more L2 cache (up to 4 mb now I think), and less power consumption. However, you won't see too much a difference if your concern is for DoD. What really makes more of a difference is the type of Graphics card you have and what the clock speed is for the GPU. That's where a LOT of the gaming difference is made.
Hope that helps.