take a look at this link:http://www.behardware.com/articles/778-2/giant-roundup-168-intel-and-amd-processors.html
You just need to know which Pent D number it is (8xx,9xx)
Since I have a Pentium D 945 3.4ghz and am looking to upgrade I have been researching it.
It is important to know your chipset since that will determine which processors you can put in.
I like Everest Home Edition by Lavalys (freeware-google it)
It tells you everything about your system.
I have been looking at the E6700 and also Q6600 since some of the Pentium D LGA775 motherboards support quad cores.
It definitley will be a nice increase going to the E6500
You should definitley notice a difference.