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 but your system won't handle it and buying a new Core 2 system now is already out of date.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!
You should be able to, but it just depends on your Motherboard as to what you can get. You already have a Prescott P4 CPU with 2MB of RAM and Hyper Threading. A true dual core will certainly preform better in multithreaded apps but the improvement may not be that great and would still be slow by today's standards (you're talking about 6 yr old tech after all.) If your board allows you to support the older Core 2 Duos get one of those. If not, then find out the best it does support and see what you can snag on eBay.
If your board supports the 1066FSB (which would make it the 925XE), then the best Pentiums you can get are the dual core EE editions. There is the 840, 955, and 965. They have 4 threads since they are dual cores with hyperthreading. The 925X boards are limited to an 800FSB and DDR2 533.