if you don't want retarded bass, you can still get great sound from upgraded interior speakers. you'll want t nice 4 channel amp to ensure that you're getting the best sound possible. of course none of this can be accomplished without a solid head unit.
You can honestly spend as you want on this. The sky is damn near the limit. It's not necessary to spend thousands of dollars, but for high end stuff you could easily break the 1,000 barrier for a head unit, 4 channel amp, and new interior speakers.