which stream would i choose for becoming an IAS officer?

You can choose any stream in order to become a IAS officer.

Nevertheless, arts/humanities helps one with the kinds of subjects that one gets exposed to (like geography,history,sociology,psychology,languages etc)

To read more on the career in IAS log on to http://www.youngbuzz.com/careers/careerdetail/civil_services_officer

This will help you know the career scope,opportunities, remuneration, career map, benefits etc.