That's what so great about an audio upgrade, its all good. You can do whatever you want, spend as much as you want. Tons of good stuff out there.
If I wanted a fairly cheap upgrade I'd just replace the speakers. But be careful, high power speakers will sound like crap when connected only to a HU. 50w speakers will sound much better for that application.
For a $100-200 more you can add a 4 channel amp.
The sky is the limit from there.