Are you listening only to radio stations? Are you listening to AM radio stations. AM just about always sounds bad because the antenna's on modern cars is tuned for FM. AM reception is minimal and AM is more susceptible to noise.
If your other inputs such as CD, iPod or thumb drives is bad, then you best bang for the buck is the speakers. My personal favorite speakers are Polk Audio, but speakers can be subject to personal taste so go to an electronics store and listen to various models for yourself. Odds are your favorite will be different from mine, but at least do try them in the store.
If you need a new head unit, you will need new speakers as most factory speakers cannot handle the power of even the weakest aftermarket head units, so start with the speakers and that my be enough. If it isn't, you would have had to get them anyway so nothing lost.