What are the best lowering kits, and exhaust system for a 2002 2.5 Nissan Altima?

Tom. First I would get the Sprint spring, google sprint spring, It lower the car 2 inches. I recommended changing the shock absorbers to tokico brand. as for the exhaust, get the magnaflow 40 series muffler. look at your stock muffler, what is the inlet and outlet size of the pipe, also are the inlet and outlet in the center or corner of the muffler. go to magnaflow.com. It's costs less than hundred dollars.You don't need cold air intake. get the k&n drop in filter. You get the same sound and results, and save a lot of moolas.

moderate lowering choices are Eibach Prokit or Tein H-Tech (no camber kit needed). more aggressive lowering choices are Eibach Sportlines or Tein S-Tech (camber kit recommended). there are other springs available, including H&R, Vogtland, Zenn. Tein and KSport also make coilovers. depends how much you're willing to spend.

regarding exhaust, don't forget that adding intake also has an impact on the sound. there are many combinations of intake/header/exhaust available to you. this link should keep you busy for a while (has examples for both 2.5 and 3.5): http://www.nissanclub.com/forums/engine-performanc...

i highly recommend joining a nissan forum. you can find answers to your questions and discover many ideas that you haven't thought of yet.