No, I don't think Saruman could defeat Sauron. Both Sauron and Saruman were Maiar of Aule, but Saruman was put into Middle Earth around 1000 TA, whereas Sauron had been around long before the First Age. During which he was the right hand man to Melkor, and his most feared servant. Sauron was known as probably the most powerful Maiar -ever-, and although Saruman was the oldest of his and Gandalf's order, he would not be powerful enough to defeat the Dark Lord.
About Saruman throwing the Ring into Mount Doom... I don't believe he would. It was the study of and the obsession with the Ring of Power that corrupted him, and if he did get his hands on it, I believe the very last thing he would do would be to throw it away.
Without the Ring he would not have the ability to take over and become the "supreme super-evil power", just as the power of the three Elvish Rings diminished, as did the Ring Wraiths.
Also, in the end of the Two Towers (the book anyway) Saruman is driven out of Orthanc by Gandalf, and I think it's clear that Gandalf couldn't defeat Sauron on his own.
Hope this helps!
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**~You see things; and you say, 'Why?' But I dream things that never were; and I say, "Why not?" --- George Bernard Shaw~**
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