That's a good question.
My understanding is the program you are using. If you use photoshop, for instance, it is optimised for dual processors and will be quite a bit faster than a single.
Reports indicate, however, non-optimised programs can actually be slower on dual processors. I think the example they used was a large Excel spreadsheet. I'm not sure about this, but I also seem to recall that Jaguar and Panther are NOT optimised for duallies. Please, someone correct me if I am wrong