I see that the Pentium D is a 64-bit processor, but when I search for Intel processors with 64-bit registers, only the Itaniums are listed. What is the register width of the Pentium D processor? Also, know of any good websites that provide information like this for processors?

Well, the register size depends on what kind of registers you mean and what mode the processor is in.

In full 64-bit mode, x64 processors have 16 64-bit general-purpose registers plus the floating point and multimedia stuff. You can still use all the registers with less than 64 bits of data of course.

And, I know this may come as a shock to you, but Intel for example actually supplies documentation for its products
Like, when you look under "Technical Documentation", you may find a page like this: http://www.intel.com/products/processor/manuals/index.htm