My first comment here
!
I think that Saurman thought he could become Sauron, if he could get the Ring, and that perhaps he joined with Sauron in the hopes that he might get information that would be of use to him. I think Saruman intended to use his allegiance with Sauron as a cover for his true intensions of trying to get the Ring. I also think that in the films they should have shown Saurman as more of his own force rather then just a poppet of Sauron.