Should I finish my bachelors degree this summer with a major in Sociology...?

Unless you plan on using the psychology degree for something, my answer is no. You state that you enjoy doing research, I think you would be better off continuing your education with at least a master's degree.

Most research firms look for people who have advanced research skills in the social sciences. They want to know that you are able to do both quantitative and qualitative work. A master's degree in sociology would help you enhance these skills. I am not confident that continuing undergraduate studies would give you the same opportunities. Good luck!

If your parents have a lot of money and you have the time - yes.

If they do not. A degree is just a ticket to get a job. Doing well, good experience and a good work ethic is what distinguishes you from the rest. At this stage in your studies, you can acquire the additional knowledge just by reading.

Go for the double major adding psychology. The extra semester won't kill ya =)

Well, it depends. Do you ever plan on using psychology or incorporating it into your work as a sociologist?

Or do you not want to do either of those and just want to beef up your resume? Focus your goals and you'll achieve much much more.