A 1.8ghz C2D will be ~= a PD @ 3.6Ghz, so in this case, the 2160 will be undeniably faster. Pentium D is also not nice to have above 3ghz, as they get *very* hot. Back in the PD days, unless you were bargain-shopping with the 805 overclock special, X2 was the way to go. My how things have changed. Make sure your board will run the 2160 though. Btw, the 2160 will save tons on power/heat. They even tested one with the fan unplugged and it still performed flawlessly. I would think the D 925 would smoke itself / shut off within a very short period of time in comparison.
You would also be well served by an X2 3600+ for your uses, it's very cheap, and since you don't game, the minor performance difference won't matter. The cheap Nvidia-based mobos for the X2 are very good, much better than the cheap C2D mobos.