I have talked to a bunch of buddies with winches, and so far, most have said that 6000lb is ok if you use a high amp alternator. Expect double line pulls most of the time with the 6000lb.
8000lb winches are more than capable of getting Jeeps out of trouble using single line only .. double line for the crazy stucks.
Anything over 9000lb is overkill on most Jeeps .. unless you are running 40" or larger tires. You are more likely to rip your frame in half with a 12,000lb winch if one end is tied to a tree and the other is pullin' a buddy out.
You will want the lowest profile winch possible to keep the airflow to the radiator maximum to prevent over heating.
Make sure to build a "winch kit" that includes gloves, tree saver, cable dampner, shackles, snatch block .. visit Warn.com
and click on their accessories listing .. they do have premade kits available as well.