Well, as you probably know, a sound system is just that--a system. If you replace just the head unit I doubt you'll be happy. Speakers in addition are probably a great idea, but I have no idea how good the stock amp is.
I'll bet the subject has been beaten to death before so I'd recommend sniffing around these forums and others for more info.
Of course, I'd also throw my hat in the ring for
Reus Audio, but they're not cheap.