drive to your nearest BIG Halfords with a CD, [with Not too much swearing in the Rap], and spend some time listening to the different speakers in your size,
If you have a good ear, you might notice that some speakers are designed to sound REALLY good ad medium volumes, and some only come to life when they are cranked up [probably due to the thickness of the rubber surround & weight of cone].
Then go Price shopping on the Net.
Wayne had a good point about the Frequency Range. But if you have got knackered door speakers, its STILL definately a good place to start,
factory speakers on most cars are made of paper, ad degrade after a few years.
Decent speakers will sound better then the standard items straight away, (with a half decent Factory headunit), i think Components are a bit expensive tho, considering Co-axial speaker are quite good these days. (i have Vibe 5& half inch Co-axials), [but the 6" Vibe didnt sound as good as a differnt brand - cant remember what it was]
i've noticed that Components only really step ahead of Co-axials when they are hooked up to a seperate Amp.
Or if the placement of the speakers is bad (ie, pointing at your foot), then Components can hel alot with sound staging [making the sound appear to come from where you want it to, instead of your Feet]
For those who dont know, Components have a seperat tweeter to the MidBass Driver/speaker. Designed to be fitted seperately, but near to eachother.
Co-axials have a tweeter mounted infront of the midbass driver/speaker, Designed to fit in the same space as a standard speaker.
Dont worry too much about sticking to one brand, but it is important to look at the RMS ratings.
Most imprtantly,DO NOT listen to the salesMan's idea of what speaker sounds good, if you are able to listen and make your own choice. Many sales people are biased to the product that has a higher profit, or towards the one they fitted in their Nova. LoL
music aint about facts of figures, it about what makes you feel good.