Just today i got XM radio installed at the circuit city that i work at. it works great and i love having it. right now my audio setup is a sony xav-7w 6.5" widescreen with a jvc kv-dv7 dvd/cd receiver on the bottom in the dash, plus the xm. however, i have the stock sound system, and the stock subwoofer is not working with this new setup. i am looking at replacing the stock speakers and putting in a new sub. two questions:

1) what size speakers should i use in the front and rear?

2) can i put a different subwoofer in the same location as the stock? i don't want to sacrifice any trunk space if possible. what other options do i have?