A good article on dual core cpus vs two cpus can be read here.
The answer to your second question was found here.
Table 1-1. Key Features Available Only in Windows XP Professional
Support for multiple processors Windows XP Professional supports symmetric multiprocessor (SMP) designs that employ up to two CPUs. If you install Windows XP Home Edition on an SMP system, it will not use the second processor.
Google is your friend, try it.