When Saruman met with Gandalf in Rivendell, he had for some time been seeking the One Ring, but he had not made any plans to ally with Sauron.
The events of The Hobbit take place in the year 2941 of the Third Age. From Appendix B of The Lord of the Rings we have this:
The White Council meets. Gandalf urges an attack on Dol Goldur. Saruman overrules him.1 Saruman begins to search near the Gladden Fields.
1 It afterwards became clear that Saruman had then begun to desire to possess the One Ring himself, and he hoped that it might reveal itself, seeking its master, if Sauron were let be for a time.
So from this we know that for some 90 years before The Hobbit (and the meeting with Gandalf), Saruman was actively seeking the One Ring for himself. As to whether he was planning on allying himself with Sauron at the time of his meeting with Gandalf, we have the following also from Appendix B:
Saruman discovers that Sauron's servants are searching the Anduin near Gladden Fields, and that Sauron therefore has learned of Isildur's end. He is alarmed, but says nothing to the Council.
Thorin Oakenshield and Gandalf visit Bilbo in the Shire. Bilbo meets Smeagol-Gollum and finds the Ring. The White Council meets; Saruman agrees to an attack on Dol Goldur, since he now wishes to prevent Sauron from searching the River. Sauron having made his plans abandons Dol Goldur.
When Saruman met with Gandalf during the events of The Hobbit, he in fact was still planning on getting the Ring before Sauron and becoming a single independent power. It would not have been until some time later (probably shortly after 2951 when Sauron declares himself openly in Mordor) that Saruman decided to side with Sauron instead of trying to get the Ring first (it was in 2953 that Saruman withdrew to Isengard).