[GregK]

It's not just the speakers, it's also an amplifier. The factory speakers are 2 ohm (Fosgate or not) and that means you'll need an amplifier as soon as you change them because all aftermarket speakers are 4 ohm.
Without an amp you'll cut the volume of the factory system in half as soon as you swap the speakers out.

 

[04titanoffrd]

How heavy of an amp do you need? Can you hook it up to the factory stero?

 

[GregK]

Any amp will do depending on sound quality and volume that you want. You can use a line out converter to connect the amp to the factory system.