There are some great solutions for you if you want to keep your head unit stock to keep all the features it came with and still have great sound. Some products will sound better than an aftermarket deck!
Just amping your speakers AND using a good product like JL Cleansweep, or AudioContol LCx will give you a HUGE change in audio quality. I recommend changing your speakers too if you're going this route.
Check out your local stereo shops, not big box stores like best buy.
Just installing new speakers wont help if the signal from the OEM deck sucks. There is just not enough power from it to make it sound good.