I got Polk's to put in my wife's Eclipse. Haven't put them in yet but so far they work great (Heh Heh).
I haven't heard of anyone that upgraded speakers without upgrading the rest of the system.
Look on the right side of the home page (near bottom) there is a "how to" section. ETLSport posted directions on replacing the rear speakers. Rumor has it that it's a real PIA.