They would come out pretty close. I would go with the 1GHz because you can always dual it off later with a dual board and a second CPU, whereas the 500MHz CPUs are getting to be stale technology. You would also have more flexibility in operating systems, ie you can use Win9x. There is supposedly a slight edge to a dual system because the operating system leaves the second CPU pretty much alone, allowing for a slight edge in efficiency, but with 1000MHz to play with, I think that is just a drop in the bucket.