Well, what are you looking for... Hi-fidelity... or the simple ability to play your MP3 Files? If you want Hi-fi quality sound, you wouldn't play 128-bit encoded MP3's. But we're talking about a car... road noise and wind will overshadow any flaws in the sound at a respectable volume... and from the sounds of it, you have an OEM deck and likely with OEM speakers.
Now, I have a deck that plays MP3, WMA, and AAC/M4A audio once put on a CD-R/RW. It works great. I also carry a flash player that I USB connect to the FM modulator for stuff I haven't gotten around to burning to a CD, it's fine, but the sound quality is not as good as something ripped to a CD-R, but it's not bad either. But considering I spent less than 30 bucks on it, it was a steal.
Go online and look at the product manual before you buy to see if it's right for you.