I wouldn't bother replacing HU without a speaker upgrade though.. Stock speakers are absolute garbage.. Just really really bad unless you have the "upgraded" stock speakers that are the two way silver looking things which I guess are pretty good from what people say.
Polk DB are a good cheap replacement, but I had them and upgraded to Alpine SPR-60
and the difference is pretty big in sound quality. For a coaxials they are pretty damn good. I have them amped with 150w RMS each up front though. I think they need an amp. If no amp, maybe the Polks would be better.