What motor? What transmission? What are your main goals and uses for the car going to be?
My ls3 responded very well to ARH long tubes, catted high flow x-pipe, corsa sport axle backs, cold air intake (lots of choices...mine was airraid), elite catch can, plug wire sock set. Then add a dyno session and a tune. Also the michelin pilot supersport tires made it a whole new car. Depending on what you have, a heads and cam pkg and maybe an intake as well might be what you are looking for. I'd suggest trick flow heads (more of a must do on an ls2 as ls3 heads breathe a bit better) and Brian tooley for cam stuff. Could also beef up what ever tranny u have (3xxx rpm stall for an auto as well is fun) axles and diff too. Hope this helps.