if this is your car -> (most 2002s, all 2-liter 320i's, 1.8L 320iA's) then you have a 3.46:1 differential. anything with a higher ratio than that would increase torque and 0-60 times.
i wouldn't recomend anything higher than 4.2:1, especially if you drive a manual, you'll redline much quicker with the 3.46:1.
<img src='http://www.bmw-forums.com/forum/uploads/post-46-1095116979.jpg' border='0' alt='user posted image' />
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