Depending on how far you want to go,fit a small external amp, hide it somewhere (in the glove box, under the seat, in the boot etc) and get some decent make same size speakers, sony, pioneer, jamo etc depending on your personal preferences -some speakers cater more for specific types of music - ie jamos great for dance music (leftfield, chemical brothers etc) but useless for rock/pop tunes (oasis, kaiser chiefs etc). quite cheaply You will find that your music is greatly improved and is a lot clearer and richer, and everthing will still look factory.
Or just get yourself some good old 6X9s, a nice improvement in sound for what they are.
Ps get some good oxygen free cable aswel. (good head unit, good connections + good speakers= good quality sound).
Like i said at the the beginning it depends on how far you want to go, and to what level you want to listen to your music, some people want the whole Dolby true digital surround sound 7.1 experience, others just want to hear noise from thier radio.