One of the biggest expenses on my upgrade list is getting a new ring and pinion since the current one is pretty tall. Changing it out is a bitch since my car has a transaxle and the new ring and pinion are going to set me back over $1K.
As FordMan already mentioned it will do wonders for my acceleration.
Right now my final ratio is about 2.5 but I'm planning on changing to 3.2