All C2D and Core i CPUs are x86 CPUs with 64-bit instruction extensions.
Just because you can physically install the Xeon CPU does not mean it will be recognized by the motherboard. The motherboard has a BIOS which basically allows it to recognize CPUs. If the BIOS does not contain any information pertaining to the CPU you want to install, then your new CPU will not be recognized and will not work.
As stated before, go to the motherboard's manufacturer website to find a list of compatible CPUs. If it is listed, then you may need to flash your motherboard to upgrade the BIOS.