As far as the front center channel speaker goes, I think the stock speaker is 4 ohm. You are going to want an 8 ohm replacement woofer for that location.
Now, the only real issue that I think most of us had was finding a door speaker that would clear the internals of the door. Depth is the only real issue and getting the mounting holes to line up with the replacement speaker that you choose. I did have to drill a hole or two to get the Kicker CS to mount flush, but then again when it comes to electronics and audio I'm about as anal as they come.
I ended up with a complete Kicker system by the time I was done.... 4 door speakers fed by a Kicker 4 channel amp and a 12" Kicker CompR with a Kicker mono amp.
I know Kicker isn't the best out there, but for a decent hard pounding system that didn't eat into my parts budget for the car.... I don't complain..... but I bet my town does when I roll in blasting Korn or Rob Zombie.
Thanks for the heads up on the center speaker.
I'll have to look into the Kickers some more. I don't have to have the best out there so saving some money here to put into other areas could be a good option.