How far do you want to take your quest for better sound? "preium" sound systems in "cheaper" cars always make me LOL... Some aren't that bad, but most suck as bad as any other... There are 3 places you could start, as Kami has already eluded to... HU, amp, speakers.. With your particular system, I would consider getting a components set (mid/tweeter with outboard passive crossover). Components with the passive crossover afford you much better sound as the crossover does more for the components set in terms of splitting the frequency range better for the proper speaker (highs to the tweeter and lows to the mid) There are some really nice components out these days that won't break the bank either, the one that comes to mind right now it the Phenix Gold RSD component set... They are getting RAVE reviews for a budget components set and I personally got a set for under 100$ SHIPPED on fleaBay, although I haven't installed then yet myself, so I can't personally comment on the sound. all the reviews I've read (I do audio BTW) have been great though... Another slightly more expensive, and quite a bit nicer would be the Pioneer TS-D1720C components.. again, getting RAVE reviews for the money..
http://www.newegg.com/Product/...17238Here is a review on them from Car Audio magizine, again getting great reviews.. http://www.pioneerelectronics....w.pdfThat is a good place to start, the biggest problem with upgrading the rest is the fact that you can't really use the factory stuff, you have to bypass all of it, unless you are simply replacing speakers... It's possible, and not difficult really, it's just a decision you have to make, if you want to walk that path... Lemme know if you do..