Could Saruman have overthrown Sauron?

Yes to both counts. Saruman certainly intended to betray Sauron if he got the Ring, and indeed he said as much to Gandalf, saying that he would take the Ring and overthrow the Dark Lord, ruling justly and wisely in his place.

What if Saruman joined Gandalf?

If Gandalf had joined when Saruman propositioned him, the Council of Elrond would have been without Gandalf and the Fellowship (if there was one) would have been either killed by Saruman and Gandalf trying to pass the Gap of Rohan or killed by the balrog in Moria.

What would happen if Saruman got the Ring?

With possession of the one ring he would have replaced Sauron (would have wiped him out entirely). Saruman wasn’t an elf, or even a man. The one ring would have made him more like what he was before he left on his mission to middle-earth, only corrupted and evil. He easily would have had mastery over the ring itself.

What would happen if Gandalf had the Ring?

However, Gandalf knew that if he — being an extremely powerful Maiar just like Sauron — had taken the One Ring, he would become no different than the Dark Lord. Thus, while Gandalf taking the One Ring would have expediated the destruction of Sauron, it also would have hasted the subjugation of Middle-earth.

Why does Saruman help Sauron?

Saruman’s real intention was to permit Sauron to build up his strength, so that the One Ring would reveal itself. He later found that Sauron had more knowledge of the possible location of the One Ring than he expected, and in TA 2941, Saruman finally agreed to attack Dol Guldur.

How is Saruman turned towards evil?

Curunir studied too long the arts of the Enemy and began to admire him as a rival and not a foe. Then he left the path of wisdom and fell from his high purpose. Basically Sarumn delved into the dark knowledge of Sauron and his arts and slowly began to admire them. That was the start of his long fall into evil.