Only replacing the speakers won't do much to improve the sound. You'll still have the same signal coming from the radio/factory amp. It might sound slightly better but by no means would you be getting a good upgrade.
I think 13Sonata2 is asking for decent brand of speakers (as replacement) that matches the dimensions (and specs) of the existing Infinity speaker in the dash so that there is not much problematic retrofitting?
Any chance you can share with us the diameter of that speaker in the dash? How about its power rating, sensitivity, impedance (Ohms)? It is always a good idea to get those brands that have very close specs to the existing speaker, such that you only want to get an upgrade with better speaker material thus giving better sound reproduction (clearer, crisper/better mids, better soundstaging, palpability, etc.).
Although, Tenergy is right. That Infinity speaker in there is no slouch so perhaps a notable upgrade would come from the stereo/source or the amplifier.