There's two speakers but I believe the vert rear speakers are 5 1/4 instead of the usual 6"x8". Front ones are 6"x8" though.
I used this site when I was doing my rip out; it helped me so it might help you. Even has some vert topics so be sure to take a look.
http://www.flemworld.com/Mach460/Mach.htmlJust curious, how much are you looking to improve your soundstage? Just want to throw in some better speakers and be done with it? Or do you want to go one step further? If it's the latter, I can help because I recently installed mids and tweets in an active setup and of course an amp to provide clean power.