well if ur gonna upgrade ur speakers, u should upgrade ur radio too, the stock radio wont have enuf power to drive good quality speakers, you might also need an amp to power them, its a costly investment. I dont even wanna think about how much ive spent on audio stuff for my car, well over 2k. For the front i suggest a component set, depending on ur budget it can vary from like $150 to over $500, I just bought infinity kappas, that normally would cost 300 but i got em for 150 off ebay,
http://www.crutchfield.com/cgi-bin/S-imQ5e9xZDaC/ProdView.asp?s=0&c=10&g=76400&I=108K605CS&o=p&a=1
the rear you can get any 6x9's since you wont really hear the rear speakers as much i got those pioneer 6999's they have a horn tweeter for better highs and a good woofer for bass response, but if you wanna go for a budget set up try getting a radio for like $150, fron components for around $100 rear 6x9s for under $100 and an amp to power them
heres a setup i found that you can do for under $600
Infinity Reference 6x9's $62 100 watts RMS
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1380212838
Infinity Reference 6.5 Component set $105 90 watts RMS
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1380212823
Pioneer P4400 Head Unit $149
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1378254411
Us Acoustics 4086 85x4 amp 189
http://www.crutchfield.com/cgi-bin/S-imQ5e9xZDaC/ProdView.asp?s=0&c=3&g=120&I=235USX408B&o=p&a=0
sorry couldnt find it on ebay
That comes out to 505 plus shipping, also you will need cables which will cost ya around $100 i think[/url]