SPEAKER SIZE?? AND WHAT TO GET??

I have a 1997 GT Convertible. It has a total of eight speakers in it. It's the Mach 460, but I have a Alpine head unit it for right now. I want to change the speakers of the small ones "think 3" ones" and also the bigger ones which are 6x8?? I want to change out the front and back. I was told that my year 460 can hold component speakers like Infinity. Is this true? I'm trying to figure out what size and what good quality brand I should get. thanks