The Pentium D was crap the day it was released. If your mobo supports them, a Core 2 Duo (or Quad) would be a MUCH bigger upgrade.
Yeah get a C2 Duo or C2 Quad for huge performance gain.
Your performance may even degrade, as, last time I checked, CrossFire only uses a single core. The Pentium D has two "weaker" cores over the Pentium 4's single "stronger" core. This is assuming you have a Pentium 4 LGA 775 Prescott with HT, which, the latest P4s were.
A better performance boost would be gotten by upgrading the GPU, or, RAM if you are low on RAM.
Pentium D did not have 2 weaker cores it was just 2 P4 cores strapped together ... better than P4 but the core series came out and made the Panything just a pile of ****...