I'm a fan of HD Radio more for the extra content rather than the audio quality, but there is an improvement in quality that even my untrained ears can detect. But, your mileage may vary. I do hear the difference when my radio drops the digital and falls back to the analog. One of the best examples is actually - believe it or not - during talk radio when someone calls in and is on the telephone. They sound much better on HD than analog. Go figure :b
But the sound quality you'll experience is a blend of several things in your car. Not just the head unit, but your speakers, interference from the electrical system, the quality of your car's antenna, the acoustics of the cabin, and where you are geographically in relation to the stations you're listening to. Will you experience an improvement in sound quality on analog stations just by replacing the head unit? Maybe, but it might not be a big one. That depends on how good the existing head unit is, and what you replace it with -- but I wouldn't expect a miracle here.
Are you just looking to improve sound quality? Or are you looking to add features (like the CD player and/or the ability to plug in a flash drive or phone or something?) If your goal is purely sound quality, you'd probably be better off investing in better speakers maybe. If you're looking for features, go for the features and while you're at it get a head unit with HD Radio.