There are several choices when it comes to exhaust systems too.  HKS makes three different systems for the 3/S.  The first is what I have.  It has two pipes with single exhaust tips.($750)  The tips are about 4 inches.  The second unit is just like the first one, but has dual tips on each side.($850)  HKS also makes the Drager exhaust system which is only one pipe with one huge (~5 inch) tip.($1,000)

GReddy makes an exhaust system similar to the HKS Drager exhaust. ($800)

Borla is the cheapest exhaust option and the most popular one.  The borla unit is louder than the HKS unit also.  It has two pipes with single tips.  The best place to buy a Borla exhaust is from CarParts.com.  With some of the discounts offered, a Borla can be bought for $450-$500.  You can check out my CarParts page at http://www.mn3s.org/car-parts.html

ATR and Blitz makes exhausts for the 3/S but I'm not that familiar with them.

HKS drager exhaust

Borla TT exhaust

GReddy PE exhaust

HKS dual tip exhaust

ATR exhaust

GReddy exhaust