Some people just really feel that, even with an aftermarket head unit, you need to get an amp for "ultimate" performance.
Unless you are a supreme audiophile, I don't think that you will notice much difference in most music that you will listen to if you just do a head unit with speakers.
If you are keeping the factory head unit, just including an amp is only going to amplify whatever poor job the factory head unit amplifier is doing, and then it won't matter what speakers you are getting.
If you only ever listen to the radio, it really doesn't matter what speakers you get. If you listen to a lot of bright/clear/light music (classical, country, non-rock pop, jazz, etc), you will want to be more concerned with the mids and highs so stuff like Polk and probably Quart will be your better bets. Again, reviews can help or just bury you.
For reference, I have a 95 Subaru wagon as a daily driver. I have a simple Sony head unit ($150) and front and rear Polk speakers along with a 10" sub in the trunk. For maybe $500 total investment I have a car that can play all types of music very well and can be very loud if I want it to.
If all you really want is just something "better," you could probably just go to Circuit City and have the road shop install whatever will fit that is on sale and be happy.
Car audio is one of those things where a million people have opinions and most of them are all more-or-less correct.