This has been mentioned before but I'd really recommend that you visit
Crutchfield. Not only can you find out which speakers fit which car, they also have a lot of background info. Request one of their free catalogs -- it's nice to have something that you can flip through and make comparisons easily, and have a big list in front of you.
A separate amp isn't always necessary. It really depends on how much you want to spend. For example, I'm happy with my system, which when I bought it (gosh, 5 years ago now!!! ) cost around $400 ($250 for a CD headunit, $90 for rear speakers (6x9), and $60 for front speakers (4x6). If I had wanted to spend $200 more, I would have bought an external amp for the main speakers. $300 on top of that, and I would have gotten a sub and an amp for it. (Notice that this is all mainstream stuff from the normal product lines of some major manufacturers-- Panasonic, Kenwood, Pioneer. Not top-of-the-line by any stretch of the imagination. Most of the stuff that people have been recommending here in this thread has been pretty expensive.)
It all depends on the kind of listening that you want to do, and how loud you want to turn it. I can get my system as loud as I want it, with acceptable distortion, from just the headunit's built-in amps. I'd go for cleaner, better sound from my main speakers before I'd add a sub, though. This is just personal preference.
We had a big
thread about (car) audio a few days ago. It's worth the read.
jfall said this before, but the 'match' between your car and the speakers, and the way in which they are installed, *will* make or break the system. Proper design will do a lot to improve your results.