When I replaced the speakers in my 300m I got these bitch'n 7 Infinitys. In my TransAm I got the Sony X-plode. Both were a million time better than the stock crap cars come with.
If you're stock speakers aren't a good thick plastic, then you'll be doing yourself a favor to upgrade. My 300 had and Infinity sound head unit stock, but the speakers were not the best Infinity had to offer.