if i was in your shoes,
i would invest in a new headunit
(possilby used, can got alot better value for money)
the standard speakers in the car are by blaupunt,
so theyre not awful speakers to begin with, a nice headunit could make them sound nice,
(pioneer and clarion are renound for sound quality)
then if you want more, you can add a sub, then speakers,
rather than like wallop said, get a sub and headunit,
and within a year, replace them both for some higher quality
(the route i took)
however this is just my opinion, nobody knows whats right for you.
also, if your after quality,
forget watts, the only thing they should be used for is matching an amp to speaker,
dont think they bare much relevance to quality or loudness,
also, max watts is nothing but a figure to boast to other chavs at mcdonalds,
rms wattage is the only thing to look at,
if your really interested in getting some nice equiptment,
like wallop said again, get out there and listen to different speakers
or visit TA (
www.talkaudio.co.uk)
just dont get 6x9s or a power cap lol
nb. a new headunit will give you more setting that can be used to make a more custom sound to your liking,
also will have more EQ settings so you can match your headunit to your speakers better, finally they will have RCA (phono) outputs to wire to separate amps.
hope this helps