When you say "it" only has 20k miles, it makes me think you may be getting a full rear axle rather than just a gear set. If you're getting a full rear axle, then none of the info below applies.
If you're getting a 20k mile ring and pinion, I'm not sure it's worth the effort. Getting a used gear set to mesh properly in a new differential, without whining, is going to be a challenge. A new ring and pinion is <$200, and won't be as much hassle to set up.
Also, you may not need that pinion bearing linked above. Your car has pretty low miles on it; the shop doing your swap should be able to press off your existing bearing and re-use it. We did that on my 13, with about 3k miles on it. If they're careful and go slow, it can easily be done.
3.73s are definitely worth it, assuming you're running a 3.31 or lower (numerically) gear now. Dollar-for-dollar the best seat-of-the-pants performance improvement you can buy.