Alpine is really good, but if you want to go a bit cheaper, you can get a kickass Pioneer deck for around 100. I personally have
this one. High wattage, interface is simple, and its got Ipod connectivity/remote/USB/MP3 CD compatability/randomthingyoudonteverneed..
I did notice a small drop in bass over my standard deck, even with bass boost on, but that's why I bought myself a RF Subwoofer. Its got outputs for F/R/Sub too, in case you ever want to "Expand" your system.
Also, if you want better sound, I would definitely replace your speakers, a new deck will overpower your stock ones, and youll start to hear buzzing and rattling in the heavy bass notes. Speaker wise, Pioneer is cheaper, but if you feel like splurging, Rockford Fosgate makes kickass speakers. Those would be my 2 suggestions.
And, DONT BUY RETAIL.
Car Audio Stereo - Car Subwoofers - Car Amplifiers and Speakers... Between my roomate and I, we've spent about 2 grand there, and haven't been dissapointed. At all. It's like the newegg of car audio.