Mobile Audio Systems & GPS

[Terra517]

Hi everyone! I'd like some advice please?

I want to upgrade my car's audio system to something better than what it has. What I'm looking for is maybe better sound but mainly something where I can plug an mp3 player (or cell phone if there's such a thing?) in and play it through my car speakers.

The car I have:
Toyota Corolla S 2008 model.

(I also noticed that my car has this problem where when I go over bumps the CD would skip or stop for a second and then continue to play and it doesn't read CD's that are just a bit scratched unlike my dad's car audio system that reads CD's even if it's a bit scratched up or go over bumps on the hwy...the difference is that my sound system that my car came with sounds better than his. So I might need new speakers then? Or do you think I just need to upgrade the In dash CD Deck?)

If I want to upgrade the speakers but mainly the 'In Dash CD Deck' (not sure if that's what it's called but I was looking at bestbuys JVC and Kenwood brands).

I'm afraid of getting something that's not as great as what my car is made with? Can someone advise what might be best to get for this kind of car?

*Does changing the 'in dash CD deck' change the sound?

*What is best for me to get if I want to play an mp3 player and CD's? This  This or this?  or anyone have any recommendations?

Thank you in advance!

(I posted all this but I'm not sure if Best Buy's Geek squad can help with this kind of audio car problem? I'm new at this. ^_^)