IMO any aftermarket speaker is better than that of a factory speaker. The main difference between the two is the materials used. In many stock speakers all you have is a paper "cone" with foam surounds. What I mean is the driver or surface of the speaker is only paper and the outer layer surrounding this paper is usally a type of cheap foam. I can tell you from experience in my truck that this foam stuff doesn't like the enviroment of the interior, it falls apart and makes a big mess.
Aftermarket or replacement speakers are usually made out of more durable materials. The surrounds are often rubber which is more durable and gives a better punch to the bass, and the driver's materials are also stronger and produce a better sound.
One of the biggest changes between a standard stock speaker and a new speaker is that the old speaker is all on piece with a little raised hemisphere in the middle. Where the new replacement actually has a seperate tweeter driver. (I do not mean seperated, it is still attached).
New speakers can handle more power before they distort, because they are made to handle it. Even your most basic cheap set of aftermarket coaxials will sound better than your original speakers, but of course for the majority of the time, the more you spend... the better product you will get.
Everything that I have currently replaced in my truck has come from www.crutchfield.com. I have had no problems with their service or products. And it is easy to match specific components to your car. I suggest you read the information on their site as well as the forums at: www.sounddomain.com.
I hope this helps, and yes I am really tired so if i mixed something up, someone please correct me.
[ 10-13-2001: Message edited by: CaffeineAddict ]