Agree with the Alpine advice - very good sound and build quality along with a red backlighting that matches the standard coop interior perfectly.
However, as the NatWest ads says
, "there is another way" regarding speakers. If you leave the standard speakers in, the upgraded head unit will often make them sound much better than the stock unit. If you were to then add a decent sub and amp combo to the rear of the car, you would be adding much needed bass which will really make your system soar and add real soul to it.
The above is dependant on you wanting to build a "proper" system. If you're just looking for a "quick fix", upgrade the fronts with a decent set of components.
The sub / amp advice came from Steve at Mega-Watt and this guy really knows his stuff.