I use Polk in my home theater and Infinity Reference in my Challenger, but then again I have separate amps in my Challenger to drive everything.
Almost anything would be an upgrade over the stock paper cone things they call speakers. My recommendation is to have an overall plan for what you want to accomplish and don't just throw things in here and there.
I added a powered sub (Infinity BassLink 10") to beef up the bottom end of the stock sound system, and it was enough of an improvement for me that i put everything else I had planned to do on hold indefinitely (upgrade all speakers and H/U). That was a year ago and I'm still happy enough today with the sound that I've taken the other upgrades completely off the To-Do list. Maybe some time in the future, but not the near future.