HiFi
As has been said already, it all depends on what u want.
U can either go the way of seperate amps and subs, but these can b quite costly and require good installation knowledge or u can simply upgrade the existing components.
I went 4 the latter and put a pair of Sony 4-way 200w 6x9's on the parcel shelf, using the wires from the factory fit rear speakers. I then replaced the door speakers with a pair of Pioneer 5.25's and the dash tweeters with a pair of Kinetic silk dome tweeters. I swapped the original head unit 4 a Sony 650 CD Unit and that suits me down 2 the ground. Very simple 2 install, quite cheap (Head unit £160 approx, speakers about £100 all 2gether). This gives me a good thumping bass, nice mid range and a great top end. Sure, it's not as bassy and as powerful as a amp & sub install, but it sounds good and I can whack it up with no discernable distortion.
At the end of the day, it comes down 2 budget and personal taste. There r loads of people on here who have huge ICE installs that would b more than happy 2 point u in the right direction regarding equipment and prices. Also, try www.fiatforum.com and go 2 the ICE section. There's some good tips in there.
All the best,
Rob.