I have a core 2 duo (E6400) laying around (out of a dell e520) and my dad has been wanting a bit of a performance upgrade in his old dell. The current processor in the dell is a P4 524 @ 3.06 Ghz. I know that they are both socket 775, but the question is, will a motherboard built to support a p4 handle a c2d? Aren't the voltages higher in a p4? Figured I'd ask before I scrap the entire motherboard.
Socket T (also known as socket 775 or LGA):
Not all existing LGA 775 motherboards support dual core Pentium D and Core 2 Duo processors...
This is a very good place to start as a guideline.
You can download SIW (freeware) to identify exactly the make, model and version number of your motherboard to research its CPU compatibility. I would download the standalone version to a flashdrive as it can be run from the flashdrive on any pc without installation.