I wouldn't consider going from a P4 640 3.2GHz with HT to a Pentium D anyways... the money you waste on a second hand Pentium D, even if it's only $20... could be better spent on RAM or cooling for a new build later. Pentium D is just 2 Pentium 4 chips stuck together and they still communicate with each other at the system FSB.
So it won't be much faster clock for clock unless you would get the highest Pentium D... in that case would also need to upgrade your CPU cooler cause putting 2 Pentium 4 chips together in one case only makes heat go up.
Your chipset does not support Core 2 chips. They have both cores and 1 die and really are worth the money... but your system won't handle it and buying a new Core 2 system now is already out of date.
Save up a little dough and go with an Athlon x2 or x3 setup with 4 gigs of RAM. If your P4 is good enough for your currently daily tasks, just keep saving money for that future build. And if you know a little about system building, then don't go OEM next time. Then you do it yourself, you have more control and more possibilities for future upgrades!