this is a big debate, but IMHO it's better to spend twice as much money upgrading with front separates and keeping the stock rear 6x9's as rear fill than spending half your money on fronts and half on rears. most of your staging and imaging comes from the front. the more accurate your soundtage is, the better the overall sound. the rear does not need to be nearly as accurate. and unless you want serious pounding bass, the rear 6x9s are adequate. the following are my picks for various price ranges. these are just my opinions. also keep in mind that high end speakers usually like a lot of power in order to sound good. you will need to have a separate amp to power these, any head unit with built-in amp will be inadequate.
budget:
<$200: A/D/S 236is 6.5" separates, online from full-line-audio.com (best price on the net)
<$300: MBquart PCE (better) or RCE (cheaper) series 6.5" separates, various online dealers
<$400: several different varieties from JL audio, or dynaudio GT series