It seems to me that an x-pipe is a popular choice that gives a little more volume and slightly different tone without being over powering or annoying. In my experience the best thing to do is try to find people local and listen to their setups. Although that does prove difficult depending on the car, but still worth a shot. Check out this link it has some good videos of different setups.
http://www.xoutpost.com/articles/x5/...deo-clips.html Youtube is also a good source to check out a bunch of different setups, and sometimes you get an interior sound level video as well to at least give some idea.