i'd reccomend these:
http://www.crutchfield.com/S-YBkFIFW...=0&cc=01&avf=N
Infinity Kappa 63.5i

i personally think they sound good and they're right around your price range, you might even find them cheaper on ebay. Upgrade your front speakers first, seeing as its your car and you hear the front speakers more.

Subwoofers, mids and tweeters are your main types of speakers. subwoofers produce low frequency sounds, or bass. tweeters are the other end of the spectrum, they produce high frequencies, such as voices, and mid-ranges fill in the middle frequencies. anyone feel free to correct me or elaborate.

if you like lots of bass buy a sub and an amp, or you like very vibrant loud music buy some extra tweeters. if not you dont have to have them. if you like a little uf everything just get some decent speakers.