Both 64-bit, and is a newer architecture than the pentium D, which has an older architecture (hence the pentium name).
The core 2 duo is probably better, but depending on what you use it for, you may not notice a difference.
And yes, to take full advantage of it you'll need a 64bit operating system as well as using it with a 64bit chipset.
Note though that 64bit isn't really what many think it's going to be: it won't double your computing speed, or anything like that. Really the only great substantial use for it is that you'll be able to address more than 4GB of RAM.