avoid using free air if you can.... I've seen setups w/ huge 12" free air subs that sounded much worse than you can do w/ a 6.5 enclosed properly.
On that note, check out the JL 6w0 as a sub. Its fantastic, and like I said, if enclosed properly, should sound quite nice!
Of course, everything ultimately depends on the dynamics of the car itself. What kind of car is it?