Too be honest, it all depends what you think "decent" sound is. Fisrt off, replacing the head unit with any aftermarket one is gonna sound a world better. 4x50w is about as good as your gonna get from just a headunit, and IMO, Alpine makes the best (I have an Alpine 9827, and it sounds great). Stay away from Sony if you can!
Yes, you are going to need to get a wire harness, unless your like me, and like to spend hours splicing wires together However, the stereo will slide and bolt right in place of the old one (i have a write up on this in the tutorial section).
The front speakers are 6 1/2's with one inch tweeters, and the rear's are 5 1/4. Depending on how much you care about your stero's "cleanness of sound", i would replace the stock speakers with aftermarket ones, preferably Infinty. Keep the stock speaker wires unless you plan on getting an amplifier for the speakers.
However, you will notice that even with aftermarket headunit and speakers,you're not gonan get great volume out of them. So it will sound good up to a point, but if you're looking for perfection or anything close to it, your gonna have to buy an amplifier for the speaks!!