The three biggest improvements you can make on a budget are the following. (in no particular order)
1. New head unit with stock speakers ($150-200)
2. Stock HU with aftermarket speakers ($100.00 to 150.00)
3. Powered subwoofer (200-250)
Of the three, the sub will make the biggest overall difference in SQ, the other two will both be noticeable upgrades with most people being split on which one is better.
The stock amp probably puts out in the vicinity of 8-12 watts per channel.
