You can check this site to get some idea of bit rates and quality.
It does depend on how your are converting the MP3s and how you intend to listen to them. In addition, it depends on your personal ability to hear differences in sound quality through your chosen playback device.
My best suggestion would be to take some specific files and do what you are going to do with different bit rates and see if they sound good enough.
Of course, the size of the resultant file might add into your decision.