first question:
vista is cheaper for the most part (i believe, i could be wrong thought). also its more about personal preference.
second question:
you can't really add up the cores, its just multiple cores. if the program is designed to utilize (use) multiple cores, then it is kind of like having a 5.2ghz cpu (if you had a 2.6ghz dual core) again, i could be wrong but i believe thats how it works.
and personally, i would stay away from ibuypower only because they have somewhat more expensive prices, and its not hard at all to build your own computer from parts.