I own an 02 civic si. I love my car. Although our cars will not be that fast with all the upgrades that you would buy. Maybe 500hp at the most. The easiest things to do would be get a nice exhaust (nice one). No fart cans. I have a greddy and it sounds very nice. You could get a nice intake. The AEM V2 is what I have and it sould pretty good. You could get a new header. Come on by ephatch.com. It's a site dedicated to the 01-05 si. Really cool and nice people. Videos and manuels and a lot of helpful information. All you have to do is register and introduce yourself in the forums. Just mention that you are looking for the guy from yahoo answers and I'll say hi and help you out. You can also look at other cars like your to get ideas. Hope to see you there.